Plywood is a great material for building the surfaces of a boat from. I've read that you used to be able to buy plywood in lengths longer than 8 feet but I've never seen any for many years now. To get around this, boatbuilders have several methods one of which is called "scarfing".

The idea is to cut a bevel through the thickness of each sheet of plywood to be joined. The width of the bevel has to be 8 times the thickness. This means 6mm plywood requires a 50mm bevel width.

It may sound difficult but it really isn't especially if you are using a gap filling epoxy. This is the setup I recently used to do some scarfs:

I'll post pictures later of the results.