I've found 1/8" and 1/4" birch 3 ply plywood at a local building center at a good price for a 4'x8' sheet.It's got a nice sanded surface on one side,but would have to be sealed really good.
Whatever it gets made of, it will need some form of protection/sealing to either protect it from the elements and/or look like you intended and keep it that way. This applies to everything from paper to sheet steel and all points between going via wood and fiberglass.
For a cargo boat that size, 1/8 for most of the skin, I assume that in your home improvement stores there is a section with small section wood strips - 1/4x1/4 makes good framing and joining reinforcement for corners. My first ones got their raw material from the panels of a dead kitchen door.