3D "polygon" collider

First, I'm a spanish speaker and I don't know english perfectly, but I'll try to explain my problem.

So: I need a collider to make a "boat" (I can't be specific), but the mesh collider makes to the boat "floats" because the water has a collider, but neither box collider nor capsule collider are useful for me, hence I need a form to edit the mesh collider or any other 3D collider as if it were a 2D polygon collider. Thanks.