Like Small Fry I know little about plywood tanks but have also built a cedar strip canoe using fiberglass & West System Epoxy. Yep, the West stuff is expensive, but apparently hard to beat. I haven't tried any other brand (just built the one boat so far), but everything I read prior to and during the build steered me towards the West brand.
As far as strength & layering, I can say from experience that my boat is extremely strong & light as well. With cedar strip/fiberglass/epoxy builds, it's actually the fiberglass that adds strength, so I'd guess that if you want to beef up the strength, get some fibreglass cloth & put the epoxy over that in a few layers, same as boat building technique. It comes in different strengths/thickness & is not that costly when compared to the epoxy. After completing the shell of cedar strips, the hull was quite flimsy & flexible. Once the glass & epoxy went on, it became a very rigid, monocoque type structure. Thwart, seats all contribute to additional strength. A few coats of marine varnish finish the job & protect the epoxy from the sun's UV rays.
Sorry, getting a little carried away with the boat stuff here, but hopefully the info will help you out with your research.
