The easiest way I can think of would be to render the boat first, then render an invisible depth-only polygon at the top of the boat (that, together with the bo

The easiest way I can think of would be to render the boat first, then render an invisible depth-only polygon at the top of the boat (that, together with the boat hull, encloses the displaced water volume), and then render the water. This doesn't add much complexity. You already have to draw the water in a pass after the boat because the water is translucent when the view angle is steep enough.