Hi @Cobra1502 -
The biggest hit to your efficiency will come from the T200 nozzle, vs. using a M200 with weedless prop like the BlueBoat employs. Nozzles on thrusters provide higher bollard (static) thrust, which is good for applications like an ROV where you often want to push hard, and not move very fast - however they are not hydrodynamic as you get going fast through the water, and will greatly increase the power required to achieve 1 to 2 m/s on a boat style vehicle…
Brushless DC motors are indeed slightly more efficient at lower voltages, as for the same power level more current is flowing, thus creating a stronger magnetic field to drive the commutation. However, the lower voltage does limit the maximum power achievable! A standard 4S battery voltage, 13-16.8V is a good compromise…