RE: Question about speed controller/battery pack
Two batteries in parallel (assuming they are the same voltage and state of charge) will give the same voltage as one. With the same load, they will last longer. Doubling the load, you are back to the same run time as one battery and one motor. The ESC will have to handle twice the current.
Over-powering a scale displacement hull is rarely a good idea - you get an unstable boat and should it survive, a shorter run time.