Interesting you mention that. I am sending some high level players off to a strange forbidding isle of my creation. That said it you can probably put in stuff like sea serpents, the occasional pirate ship and certain once in a blue moon, some strange nautical happenings. My advice for such a sea vogage, don't over do it, but if you want to make it hazardous and/or interesting, Brine hags, and probably Windriders would make for an excellent encounter.