First, get a body of cardboard to use as your walls. Then attach a base to use as the floor of your boat. Also, on your Walls, create "divots" for your arms. This will allow then to reach the water it it is tall.
Cardboard and duct tape (lots of it)
Step 1: Step 1
Attach the base to the walls using silver duct tape. Go about 2 layers over the whole boat and make sure everything is covered. After the silver tape is applied everywhere, if u want to make a banana boat, apply 1 layer of yellow tape around the whole boat.
Step 2: Step 2
Put another layer of a base in the bottom of the boat for cushion as well as support so water does not enter the bottom of the boat. Also grasp two rectangle pieces of cardboard and attach them to the front. The rectangle pieces will make the water roll off the boat other than get stuck on the flat surface causing resistance other than being aero dynamic. Duck tape these components the same way explained in the previous step
Step 3: Final Product
This is what ours looked like and I hope it works for you too. In total, it took 4 hours as well as costed 50$