at 120 amps, I would keep putting battery's in parallel just due to the motors ability to suck back the juice. Saves your packs. On top of that start with a certain prop size and keep enlarging it until your find a speed that you like. As you enlarge the prop, the motor will draw more amps to try and keep the prop spinning at that speed determined by the voltage and you will go faster.

I would recommend the avg of 14.8v(most people run their boats on) so combine them to keep that as a goal.