For those who dwell on land, the sea is an alien world filled with unpredictable hazards. Yet rare treasures hide in the depths, making such dangers well worth the risk for brave and brazen mariners. This section details a variety of environmental features one might encounter both on and beneath the waves.
A blue hole is a circular sinkhole that forms on the bed of the sea in shallow water. The deep blue water of the hole creates a stark contrast with the lighter color of the shallow water that surrounds it. Blue holes have diameters of 1d10 × 100 feet and are 1d10 × 100 feet deep.
Blue holes are filled with secrets. Many blue holes hide monsters or treasure.
Coral reefs grow all over the ocean floor, though most are found within depths of less than 150 feet near the shoreline. Reefs range from a few feet to over 1,000 miles in length and width. These colorful ecosystems house small fish, crustaceans, and mollusks. As a result, predators come to reefs looking for prey.
Uneven and sharp, coral reefs can harm those forced into them. A creature forcibly moved on the reef—without using the creature's action, bonus action, or reaction—takes 3 (1d6) slashing damage per 5 feet of reef that it is pushed over.
Ocean water is moved by wind and tides to create constant river-like flows called currents. Tidal currents are typically the strongest. They're usually found within 50 miles of shore and at depths of less than 300 feet. These currents have a speed of 1d6 miles per hour that changes daily.
Currents become weaker farther out to sea and deeper in the ocean. These currents have a speed of 1d4−1 miles per hour that changes daily.
When a vessel or creature travels in the same direction as a current, that current's speed is added to the vessel or creature's travel pace. A creature or vessel traveling against a current has that current's speed subtracted from the vessel or creature's travel pace. If this results in a negative travel pace, the vessel or creature can let the current carry it. Alternatively, the crew of the vessel or the creature can fight against the current by rowing or swimming hard, moving at half the vessel or creature's normal pace. Vessels and creatures fighting against a current must make a Constitution saving throw at the end of every hour traveled, with a bonus equal to the crew's quality (if they have crew). Vessels that fail have their crew's quality score decreased by 1. Creatures that fail gain one level of exhaustion. The DC for this saving throw is 10 + 1 for every consecutive hour spent fighting the current.
When a creature without a swimming speed is submerged in a current, the creature must succeed on a Strength (Athletics) check at the end of its turn or get pulled 1d4 × 5 feet in the direction of the current. The DC for this check equals 10 + the number of miles per hour the current is moving.
The deeper a surface-dwelling creature travels into the sea, the more hostile the environment becomes. Those venturing into the depths must keep the following natural forces in mind.
When it comes to light, the sea is divided into three layers:
Sunlight Zone. The sunlight zone extends from the water's surface to a depth of 650 feet. This area of the sea has the same natural illumination conditions as above the water.
Twilight Zone. The twilight zone is between the depths of 650 and 1,000 feet. When the sunlight zone above is bathed in natural bright light, the twilight zone is filled with dim light. If the sunlight zone is filled with dim light or darkness, the twilight zone is dark.
Midnight Zone. Any depth below 1,000 feet is considered the midnight zone, since no natural light from the surface penetrates this deep.
Water pressure, or the weight of water on top of a creature or object, increases with depth. Conversely, temperature decreases as depth increases. The pressure and temperature's effects on creatures without a swimming speed at depths below 100 feet can be found in the “Underwater Action" section.
With this optional rule, characters who dive deep in the ocean require specialized equipment that can withstand the ocean's pressure. Nonmagical objects not made to withstand the water pressure are destroyed at various depths, as determined by the material used to create them. This destructive depth is presented for various materials on the Objects and Water Pressure table. Objects made of other materials break at the DM's discretion.
| Objects and Water Pressure | |
| Material | Destructive Depth |
| Glass, crystal, ice | 100 ft. |
| Wood, bone | 500 ft. |
| Stone | 1,000 ft. |
| Iron, steel | 1,500 ft. |
| Mithral | 2,000 ft. |
| Adamantine | 2,500 ft. |
While natural fog on the ocean can be a nuisance to ship crews, eldritch mist strikes fear into sailors everywhere. These rare, mysterious mists are almost indistinguishable from the naturally occurring variety until it's too late.
If a ship encounters eldritch mist, choose a type or roll for one on the Eldritch Mist Types table.
| Eldritch Mist Types | |
| d6 | Mist Type |
| 1-2 | Ghost Fog |
| 3-4 | Shadowfell Fog |
| 5-6 | Wild Magic Fog |
Mist lightly obscures the area it fills. Depending on its density, it heavily obscures an area beyond a certain distance from creatures within. Use the Mist Thickness table to determine a mist's thickness at random. The Mist Obfuscation table notes how far a creature in mist can see before the area beyond is heavily obscured.
| Mist Thickness | |
| d10 | Thickness |
| 1-3 | Light |
| 4-7 | Moderate |
| 8-9 | Heavy |
| 10 | Very Heavy |
| Mist Obfuscation | |
| Thickness | Heavily Obscured Distance |
| Light | 30 ft. |
| Moderate | 20 ft. |
| Heavy | 10 ft. |
| Very heavy | 5 ft. |
Ghost fog contains the souls of murder victims whose bodies were thrown into the sea. A DC 15 Intelligence (Arcana) check reveals that this mist is ghost fog.
The souls of the dead want the living to join them. Every hour a ship is immersed in ghost fog, roll any die. On an even number, 2d4 hostile specters appear and attack those on the ship. If a humanoid creature dies in ghost fog, its spirit rises as a specter that is hostile toward all creatures that aren't undead.
All non-undead creatures immersed in ghost fog are vulnerable to necrotic damage.
Shadowfell fog seeps into the Material Plane at thin points in the boundary between the two planes. A DC 15 Intelligence (Arcana) check reveals that this mist is Shadowfell fog.
When a vessel travels through this oppressive, shadow-haunted miasma, its crew and passengers feel despair. Each hour a vessel is immersed in the fog, the crew must make a quality score check by rolling a d20 and adding their quality score to the number. The DC for this check is 10 + the number of hours the ship has been immersed in the Shadowfell fog. If the crew fails this check, their quality score decreases by 1 while within the Shadowfell fog and for 1 day afterward.
Few know the origins of wild magic fog, with some claiming it is the creation of capricious gods while others believe it is the result of magical experimentation gone wrong. A DC 15 Intelligence (Arcana) check reveals that this mist is wild magic fog.
Whenever a creature in wild magic fog casts a spell of 1st level or higher, roll on the Wild Magic Surge table found here.
Kelp grows in dense forests across the ocean floor. Most forests are found within depths of less than 100 feet near the shoreline, and range from 10 feet to over 100 miles in length and width. The kelp grows in vines up to 175 feet in length.
Kelp roots are shallow, and a whole forest can be uprooted by a strong storm. But it grows at a rapid pace, sometimes as much as 18 inches a day, meaning that a kelp forest can spring up quickly. Such forests can hide thousands of small fish and other prey animals, which attract predators. The forest is so dense that anything within it is heavily obscured to an observer more than 10 feet away, and the whole area is difficult terrain.
When krakens die, their bodies often rot on the seafloor. In such cases, the ground absorbs the decaying kraken's supernatural energy, marking the area with a dark stain in the shape of the kraken's body. This stain is called a kraken's grave.
When a creature moves within 30 feet of a kraken's grave or starts its turn there, that creature must succeed on a DC 14 Dexterity (Stealth) check or it disturbs the grave. If a creature disturbs the grave, a tendril made of lightning stretches from the ground and attacks the creature with a +7 bonus to hit. On a hit, the target takes 10 (3d6) lightning damage, and it must succeed on a DC 14 Constitution saving throw or become blinded for the next 2d4 hours.
When aboleths die, their souls sometimes gather in clusters called lure lights. These 100-foot-diameter collections of pale, yellow lights are found in the deep ocean below depths of 1,000 feet. They glow with bright light in a 100-foot radius and dim light for another 100 feet.
Any creature that can see the lure lights must succeed on a DC 14 Wisdom saving throw or be charmed by the lights for 24 hours or until the lights are destroyed. A creature that succeeds on the saving throw is immune to the effect of the lure lights for 24 hours.
While charmed by the lights, a creature can't willingly move out of line of sight of them and defends them to the death. If forcibly moved away from the lights, the creature tries to find its way back to them. The creature can't take a short or long rest while under this effect. After 24 hours, the charmed creature gains one level of exhaustion and must repeat the saving throw if it is within line of sight of the lights, ending the effect on itself on a success. If the lights aren't within line of sight at this time, the creature succeeds automatically.
Each cluster of lure lights has AC 17, 100 hit points, and immunity to necrotic and poison damage.
Even worse than the threat of storms is the menace of weather imbued with magic. When a ship enters a storm, roll a d20. On a 20, the storm churns with magical energy. Pick or randomly determine the type of magical energy brimming within the storm, consulting the Magical Storm Type table.
| Magical Storm Type | |
| d8 | Magic |
| 1 | Abjuration |
| 2 | Conjuration |
| 3 | Divination |
| 4 | Enchantment |
| 5 | Evocation |
| 6 | Illusion |
| 7 | Necromancy |
| 8 | Transmutation |
A storm infused with abjuration magic repels ships, as if protecting something in the sea. Right before the storm ends, a ship within it is teleported 5d20 miles in a random direction.
When conjuration magic churns within a storm's heart, creatures of wind and rain emerge from distant realms to attack anything they encounter. The ship encounters 1d3 hostile air elementals.
When divination magic seeps into a storm, howling winds and residual whispers temporarily scramble the knowledge of a ship's crew; for 1d3 days, the ship's crew suffers disadvantage on all quality checks.
Storms infused with enchantment magic disarm a creature's sense of danger, enthralling mariners and causing them to speed into danger. During the storm, any checks made to resolve hazards or manage the ship are made with disadvantage.
These ferocious storms are wracked with thunderbolts, driving sheets of acid rain, exploding meteors, and other dangers. After each day of the storm, every component aboard the ship takes 1d10 fire, 1d10 acid, and 1d10 lightning damage. In addition, roll a d20 and add the crew's quality. On a 10 or less, the crew's quality decreases by 1, and 1d6 members of the crew are killed.
A storm brimming with illusions poses little direct harm to a ship, but experienced navigators know it poses an insidious threat. During the storm, the ship travels in a random direction that isn't its intended course.
Ghosts howl and whirl in this storm's wind, while the remains of long-dead mariners stir in their watery graves. During the storm, 1d4 specters, 2d4 ghouls, and 4d6 zombies emerge from the waves to attack the ship.
These violently unpredictable storms cause the waves to warp and twist. The storm transforms the water around the ship into an amalgamation of ice, stone, and stranger materials, making travel difficult and damaging the ship. The ship's hull takes 4d10 bludgeoning damage, and the ship's speed decreases by half during the storm.
Waves and currents deposit sand in shallow water near the shoreline. These sandbars form a line that can be between 10 feet and 3 miles in length and width. During high tide, sandbars are below the ocean's surface, while they peek above the water during low tide.
Sandbars make dangerous obstacles for vessels, particularly at high tide when they are less noticeable. The crew of a ship moving toward a sandbar beneath the waves must have a passive Wisdom (Perception) score of 12 or higher or fail to see the sandbar in the way. When a ship passes over a submerged sandbar, the sandbar counts as difficult terrain, and the ship must succeed on a Dexterity saving throw. The DC of this save corresponds with the depth of the sandbar, as listed on the Sandbar DCs table. If the ship fails this save, it gets stuck on the sandbar. A ship can become unstuck by using an action to make a Strength check, with a bonus equal to the crew's quality, against the sandbar's DC.
| Sandbar DCs | |
| DC | Description |
| 10 | Deep sandbar |
| 15 | Moderate sandbar |
| 20 | Shallow sandbar |
Sapping snow—that's what aquatic explorers call the powdery remains of dead organisms that cover the ocean floor. When this substance is imbued with necromantic magic, it becomes a life-leeching detritus.
Whenever a creature starts its turn touching sapping snow, that creature must succeed on a DC 15 Constitution saving throw or take 10 (3d6) necrotic damage. The creature's hit point maximum decreases by an amount equal to the necrotic damage taken. This reduction lasts until the creature finishes a long rest. If this effect reduces the creature's hit point maximum to 0, the creature dies, and its body immediately crumbles into sapping snow.
Shipwrecks at the bottom of the ocean make the perfect locations for monsters' lairs and lost treasures.
In areas where storms or opposed currents drive powerful waters together, violent maelstroms might form. Whirlpools are difficult terrain. Each whirlpool has a rank, which determines its size and strength, as shown on the Whirlpool Rank table. A whirlpool's depth equals half its diameter.
| Whirlpool Rank | |||
| Rank | Diameter | Velocity | DC |
| 1 | 22 (4d10) ft. | 5 ft. | 5 |
| 2 | 55 (10d10) ft. | 15 ft. | 10 |
| 3 | 110 (20d10) ft. | 25 ft. | 15 |
| 4 | 165 (30d10) ft. | 35 ft. | 20 |
When a creature moves into a whirlpool or starts its turn there, it must make a Strength (Athletics) check with a DC determined by the whirlpool's rank. On a success, the creature can move normally. On a failure, the creature is immediately moved toward the vortex's center at the whirlpool's velocity, and the creature is restrained by the whirlpool until the start of its next turn. If the creature reaches the whirlpool's center, the creature is pulled under the surface and either appears at a special location (see "Whirlpool Destinations" below) or plunges a number of feet underwater equal to the whirlpool's velocity.
If a vessel starts its turn in a whirlpool with a diameter greater than the vessel's length, the ship's officers and crew must make a group check to escape. This group check works like the special ones made against hazards.
The group check represents 5 minutes of work. The check's DC is chosen or randomly determined by rolling a d4 on the Whirlpool Rank table. The captain, first mate, bosun, and quartermaster each make an ability check, as shown on the Whirlpool Checks table. If no one makes the check for a particular officer, a failure is contributed toward the group check. Also, roll a d20 for the crew, using its quality score as a modifier to the roll, and compare that check to the DC.
Determine how many of the group's checks succeeded—the officers' and the crew's—then consult the Whirlpool Check Results table.
| Whirlpool Checks | |
| Officer | Check |
| Captain | Intelligence (water vehicles) |
| First mate | Charisma (Intimidation) |
| Bosun | Strength (carpenter's tools) |
| Quartermaster | Wisdom (Nature) |
| Whirlpool Check Results | |
| Result | Effect |
| Total Success | The vessel uses the whirlpool to its advantage and increases its speed by 20 ft. during its current turn. |
| Success | The vessel can move normally on its turn. |
| Failure | The vessel is immediately moved toward the vortex's center at the whirlpool's velocity, and the vessel is restrained by the whirlpool until the start of its next turn. |
| Total Failure | As a failure. Additionally, if the vessel is in the whirlpool at the start of its next turn, all checks the vessel makes to determine the whirlpool's effects are made with disadvantage on that turn. |
While a whirlpool might be a short-lived hazard that drags things into the depths, they might also serve as a violent passage to another realm. In such cases, things dragged into a whirlpool are deposited elsewhere, be it through a crack in the seafloor leading to an Underdark ocean or a portal to a different plane of existence. Use the Whirlpool Destinations table to choose or randomly determine where a whirlpool leads, usually to a body of water in that destination.
| Whirlpool Destinations | |
| d10 | Destination |
| 1-3 | Underdark |
| 4 | Elemental Plane of Water |
| 5 | Elemental Plane of Earth |
| 6 | Feywild |
| 7 | Shadowfell |
| 8 | Astral Plane |
| 9 | Outlands |
| 10 | Outer Plane of the DM's choice |