Ref the ply, my limited experience so far has been that ordinary 6mm ply from a timber yard is certainly usable for the sides and ends of a Springer hull, but it's a bit rough and ready compared to the stuff from the local model shop. It is half the price or less per square foot, though!
The bottoms of mine are 1/16" ply from the model shop, but for The Lady Wife's mini-Springer and the full-size one on the bench now, the sides and ends are 3mm LitePly. And tbh the main reason they're LitePly is that LitePly is the only stuff in the local shop in 2ft x 1ft sheets that isn't warped!
I do get a feeling that mine are built a bit lighter than many, but as we're not going to be playing water dodgems with ours (at least not intentionally) I simply make sure that the sides/bottom joins are all reinforced with a continuous fillet of epoxy. Personally, I wouldn't want to be using anything thicker than 1/16th for the bottom, 'cos it's a doddle to use providing you cut it the right way round.