Maybe it is possible to use the depth buffer to do the trick to some degree by drawing depth only geometry
Thanks for the tip - I could easily do that by adding a polygon to mesh with alpha=0.
This will produce other artifacts but perhaps I could dynamically enable/disable this polygon depending on boat’s position.
Perhaps the philosophy behind it is correct: you usually see boat in normal position and water shouldn’t be visible inside. When boat rolls over or goes partially underwater after a jump then I will get artifacts but that’s an out of ordinary situation anyway. So perhaps I should make the usual case better at cost of unusual case.
Look at this screenshot:
That’s the case I’m talking about - boat just landed after a jump.
With that invisible polygon I would have a hole in the water. On the other hand it’s just a few frames where boat is underwater that much.
That’s also why I mentioned that pushing the water down would be better - not difficult to implement since I have displacement allready. I could smoothly disable that effect when boat should get flooded so it’s best approach.
But I won’t be changing that right now. It’s like Serious Sam said: “Give a man a bullet and he’ll want a gun”. If I would implement that, then people will start asking for more improvements to the water 
Let’s leave the water inside boats as it is for now.