Fiberglass and balsa wood. If you have any friends with a 3d printer there are lots of 3d printable designs on the various 3d model sites like Thingiverse. You could use Styrene also. I have seen a couple styrene boat builds before. One was absolutely insane. It was like some kinda patrol boat.
This boat is just a little over $100 at $110 at Tower Hobbies, but it does almost 13 mph in this vid. Would be a great boat to do a brushless upgrade on I am guessing.