If fitting three motors, it seems a waste to have the centre one just providing drag and slowing things down. 3 ESCs and a mixer seems more logical.
Saving weight on batteries is the best way to increase performance. LiPos could be a good answer for giving the higher capacity for increasing run time.
Having got motors with enough power to do the job, the next tick is to get the right props. Initial intuition points to big, heavily pitched props with lots of blades to shift a lot of water. Usually, this just flattens batteries quickly and cooks ESCs and motors. Think in terms of small, two (preferably for performance) or three (keep most of the performance and look good out of the water) blade props. The boat can never go forwards faster than the stream of water being pushed backwards. Plenty of other factors, but thats a good one to start with.