RE: Built it, now how to make it go?
I can't tell you how to put it all together, but for some cheap electronics you could try a cordless drill. A basic 2 channel surface radio with 2 servos, a cheap cordless drill from harbor freight, and some use of your fabrication skills should get you going. You could use the drill for motor, speed control, and battery. You may want to upgrade the battery to a lead acid type for more runtime. Check out american science and surplus for cheap batteries.
A sub would make it go probably, but very slowly. One that only cost 80 would be slow on its own, without trying to push 20lbs of boat.
Hope this helps.