You're looking at this thing like you're Rube Golberg (did I spell that right?). What you need is another gear riding on the axle gear, then runs it up or down to a prop w/ a propeller. This would allow for "driving" straight into water. Then a rudder could be hooked up to the steering. It would work well with a ZZ SE. You run a second servo in parallel, then hook it up in the back on a rudder with a hinge. there ya go! Make a hull, waterproof electronics (shrink wrap), and you're set. The only problem is charging. For that, you'd have to make a wire that would "plug" into the controller. Another one of my rants, but I think that sums it up. Also, it would go into water like this car...
http://www.watercar.com/video.html