Marine ply will leech if not sealed.
Options IMO:
-Treated ply with a liner made from a water proof material(i cant find the name of it, but they make large floating items out of it, such as you climb over at the pools). They "weld" and sew the seams. Dont think its that expensive, depends who you know/see
-Treated ply with fiberglass over it
Personally id go the first option. Cheapest also i think
Its not hard to put a window in if using either method
Start collecting rain water?