Being a professional Painting Contractor I would use a product called Drylock on the interior of the plywood. I would then probably paint over the dry lock with 4 coats of a product by Sherwin Williams called Woodscape stain. It's a stain but applies like paint and dries like paint. Maybe if I'm going super crazy I would ProBlock inbetween the drylock coat and the 4 coats of woodscape.
I might even try to make a small version of a plywood tank just to see how it would come out. Seems odd but fun to do.
I'm a man and I can change, if I have to, I guess.