kind of a stream of consciousness ramble here...

watts is where its at.

for each motor to propel the same boat at the same speed (given all other motor specs are the same), they will need to use the same amount of power.

power is watts, which is amps * volts.

generally speaking the motor with the lower kv will use fewer amps to get the same watts. ie, for the motor using 7.4 volts, to get 300 watts, it will need a speed control capable of handling 41 watts. for the motor using 11.1 volts, it only needs a speed control capable of handling 28 watts. higher amp rated speed controllers cost more. higher amp rated speed controls also tend to be physically larger.

in addition, now the battery will need to be able to handle a load of 41 amps for the 2100kv motor, but only 28 amps for the 1400kv motor. higher amp rated batteries cost more. however...

generally this also means that to get the same run time out of the battery, the actual mah rating of the 7.4v battery will need to be higher than the mah rating of the 11.1v battery. this can counter-act the need for higher discharge ratings. so you might be able to buy a smaller mah rated battery, with a lower discharge rating, but with more cells. cost on the batteries will probably be a wash.