Helper,
Nothing is WATERPROOF.
Obviously, the plastic flooring didn't help a lot.
Was the plywood really rotted or just delaminated?
If it was delaminated, then marine or at least exterior grade plywood could be a workable improvement.
If it is truly rotted, you had best figure out where the water is coming from or you will be going the job again...
Interior plywood has one good side, one better and who knows what between and is bonded with water soluble adhesive.
Exterior has one good side and one not so good side and the same core, but with water resistant adhesive.
Marine plywood typically has one real good side and one great side and the core can not have voids and holes in it. It will also be a water resistant adhesive, and about twice the price of EX, but it may be worth that.
Good luck
Matt
