For our small boats, 1/4" is used on the sides and sometimes the bottom and it is the proper thickness. Think of an inflatable boat: it is the water that supports your weight, not the fabric or in this case, the thin plywood. In our larger boats and power boats, there are other loads to consider but in those cases, the hull is NOT made of 1/4" (6mm) plywood. It uses 1/4" plywood as the core of a composite sandwich: the plywood is covered on each side with biaxial fiberglass and epoxy. The resulting panel is not only 3/8" (10mm) thick but it is stronger than 3/8" marine plywood, stronger and stiffer than a 3/8" standard production single fiberglass skin. Another advantage is that the thin plywood core is very easy to bend: you would not be able to build those hull shapes from stiff 3/8" (9mm) marine ply. Altogether: easier to build and stronger: a win-win material!