I'm always puzzled with this. What do you use to create an"at sea" feel of waves? I find that all we got is "breaking" waves.
That sound has probably never been recorded before since people who are out on the open sea are usually in some sort of craft, whether it be a large liner or a small skiff or lifeboat. In either case the ocean is going to be interacting with said craft, breaking over the bow or slapping against the side. So, unless you're planning on recording from a hot air balloon, the only other way to go about this is to create it.
What type of sea are you trying to create? Stormy, calm, mountainous, flat? Your answer, of course, dictates what types of sounds you can audition. You may find the sounds of wind helpful, as well as earthquakes, deep rumbles, and gentle lake laps.
I agree with David, and have had to do this recently myself. You kind of want just some intermittent splashing going on without the white noise of water dispersal from waves.
If you want the sound of being on a boat then recording on a boat or waves hitting posts at a pier might o the trick. But if you dont want the sound of a ot then my suggestion if still just editing some splashes.
I think the best you could hope for is to use a very long boom pole to give you separation between the boat you are in and the point at which you are recording.
Jay's point about working out what kind of sea you are trying to create is also very important. The chances of you getting a perfect recording are fairly small which means you'll have to "design" a sound that will work in your context and will fit with the narrative.
In addition to the aforementioned sound of water slapping against the boat I'd also try to work in some small sounds of the vessel reacting to the waves and the movement; anything from small wood creaks in rhythm to the waves to bigger moans of the hull depending on the ship.
The SoundWorks Collection video to All is Lost (which is nominated for Best Sound Editing) is also very interesting: