Use the Weather table from the Weather Section for weather at sea.
If weather conditions indicate both a strong wind and heavy rain, they combine to create a storm with high waves. A crew caught in a storm loses sight of all landmarks (unless there's a lighthouse or other bright feature), and ability checks made to navigate during the storm have disadvantage.
In a dead calm (no wind), ships can't move under sail and must be rowed. A ship sailing against a strong wind moves at half speed.
A relatively calm sea offers great visibility. From a crow's nest, a lookout can spot another ship or a coastline up to 10 miles away, assuming clear skies. Overcast skies reduce that distance by half. Rain and fog reduce visibility just as they do on land.