Using plans found online, I constructed a surfboard using an interlocking cardboard frame and covered it in several layers of fiberglass to create a watertight cavity. The result is a translucent surfboard. 

 

The board was tested for leaks, and a small oone was found and patched soon after. A motor was added, along with a massive 'water resistant' battery, which I kept inside the backpack. I was able to ride around the Charles River at about 5 mph, which was fun and probably for the best in terms of safey.