I've got the same ESC and it can handle pretty much whatever you throw at it (up to 60 amps). Anyway, regarding motors, you definately want to stay away from car motors as they can't handle the torque that boats require. Go to offshoreelectrics.com and look at the Graupner line of motors.

Your boat probably doesn't have enough room inside to run a big motor on lots of cells judging by the pics. Maybe put in a Speed 600bb 8.4v and run it on 7 cells. Not sure where you would put multiple battery packs so that may not be an option.

Let us know how it runs out of the box.