If you attach an invisible 3D model (liferaft that is 10X wider and longer) to the lifeboat you will make it more stable in water. The game engine will use the resultant bounding box for the entire object (visible and not visible) to determine physics affects (pitch and roll from waves)
Attach a Collisionable object controller to the lifeboats and you should be able to shoot them and destroy them (sink them) also.