PET Bottle Floatation

Here's a thought (may be crazy) about a cheap option for floatation that you can fit through small holes. Polystyrene packing chips.

Polystyrene foam is closed cell and certainly stands up to seawater immersion OK, witness the vast amounts of it that wash up on beaches.

You'd need to look around carefully, as in recent years water soluble corn starch foam beads have been taking over from polystyrene, but I bet that you can still find polystyrene somewhere. I wouldn't mind betting that the stuff is dirt cheap to buy by the sackful, too.

I'm pretty sure that some bean bags use polystyrene beads, too, and the stuff is used as cavity wall insulation, although maybe those small beads might be inclined to run out of any hole in the hull, so would be best stuffed into plastic bags air inflated into the hull voids.