RE: Waterproofing Balsa?
Sealing any balsa or particularly light-ply (Which I think is made from stale saltines) is a good idea.
Polyurethane varnish has been mentioned. Thinned epoxy (epoxy+denatured alcohol) is also good, as are fuel proof dope and balsarite.
Seal the inside of the tank compartment, as well as the radio compartment. Anywhere water or fuel might lurk.
Glassing the floats or hull with a layer or 2 of 3/4 oz glass cloth does not add much weight but does add a great amount of strength. The cloth can be applied with the polyurethane varnish. Doesn't have to be epoxy.