You need to understand that for the game, there would be clipping and stability issues if you actually seated a player there. Remember that the various capes had some contact issues with the ground, etc., when seated in shallow water. Personally, I've never fallen out of a ship while walking around free, but I'm told it happens from time to time.

That's normally where the rudder (steerboard) would go in an actual longship. In a design you can spawn in but isn't in the game yet, likely to be added when they do the ocean biome overhaul, you can see its use:
https://i.ytimg.com/vi/HDLE5N9lTDc/maxresdefault.jpg for example.