Building water tight box from plywood and HDPE
When dealing with water, look at marine

People make boats out of plywood, just make one inside out.   They have several techniques, including involving fiberglass, but one of the most prominent is epoxy systems that give you a lot of options.  One such is West System epoxy.  

Keep in mind water is heavy (1 tonne per cubic meter, in fact) so your cross bracing needs to be tip top.