G'day, buildingaanoe, Welcome abard mate.
like Chuck, I too, only use the cheap 3mm ply. Over here, the real flash marine A grade stuff cost AUD $160.00 per sheet. This is way too expensive for my liking.
My cheap 'blonde' ply costs me about AUD $13.00 per sheet and while my boat is only a few months old, it has stood up very well to the type of abuse I subject it to.
Matts, plans and build instructions are complete, and easy to understand and are well worth the little expense of purchasing them. Have a look at his iroquois study plans on this site. A lovely boat with immense load carrying capacity and still a comparatively simple boat to build. there is an Englishman here who proudly speaks of completing one in a week-end (and rightly so IMHO)
good luck mate.