In old seafarer tales, sea witches were involved in the fates of ships and sailors. They controlled the seas using the moon, tides, winds, waves, and storms. Instead of cauldrons, they used what was at hand to cast spells: oyster shells, clams, driftwood, sea glass, seaweed, and hulls of wrecked/abandoned boats.
Basically, a sea witch is exactly like a witch in the woods, but with a nautical theme. I could not find any information on what their familiar would be, but one can assume they would use a seagull or a hermit crab. And I think it would be cool if the witch, instead of using a broom, rode a surfboard.