how I replaced my boats under floor flotation with foaming - YouTube.28 Nov 2013 . This is how I replaced my old flotation foam into my boat hull. Flotation foam helps a boat stay afloat if you become swamped. Old flotation can become waterlogged and unsafe. This is what I did . After screwing the new marine ply floor with stainless steel screws then drilling some pouring holes with a hole.