Here is a hint for floatation foam- look in the local yellow pages online for "Foam Fabricators" in your area and ask them if they have any scrap of a 2 lb density closed cell polyethylene (used in packaging but is very water resistant), 2 lb bun stock cross linked polyethylene (used in life jackets, PFD's etc.), or extruded polyethylene (pool noodle material). All of these have an absorption rate of less than .5%, will not support mold or mildew growth, and have very positive buoyancy. Tell them you are working on a prototype for an aquatics device. They will ususally give you scrap samples which are enough for your requirements.
They may want details of the project. Just tell them its proprietary for now and still in development. Foam guys are usually easy (I am one). If you are in So Cal, PM me and I'll hook you up.