You said "just get some big pieces of foam and put it on the fuse and on the wings.." this would lead to a lot of parasitic drag and probably kill all of the wing's lift. It would also probably make the plane fly like a dog in general. NOTE: No insult intended to any actual dog out there. Fuselages are already fairly well stuffed with things like radios, fuel tanks and pushrods that you wouldn't want to interfere with or do without. Also, aside from building a foam wing, they would be a little tough to "stuff" also. Not a practical solution I'm afraid. He would be much better off just staying out of the water completely or building a float plane