If the batteries you considered are AA size, You might want to consider AAA NiMH cells. Granted not as much capacity therefore a shorter run time, but I get well over an hour on my Tuna boat. I don't need a special charger, either. Making up your own sausage string of cells gives you the opportunity to plant their weight anywhere you need to, rather than being dictated to by the size/shape of a pack.
Losing excess plastic from the decks inside the superstructure saves some weight, but more importantly saves it where you really need to lose it, upstairs. If you then wind up with surplus buoyancy, you get the chance to use bits of flat lead in the bottom of the hull, which further helps with marginal stability.