Isn't the drill geared and the motor spins far more RPM than than the output shaft at the chuck?

This might be your cheapest possible route. Once you give it a try you may find out you could get away with less power reducing the cell count.

Not too many people build a boat that large and aim for just 30mph. I have never done this myself so I can not be certain on wattage figures. I do know that you should not need unloaded RPM speeds higher than 20 000RPM. If you go as low as even 13 000RPM you still should be fine. You will probably want to spin either a 2 or 3 bladed prop over 65mm in diameter. The kv and cell count will determine what prop can be used.

When I said 1200 watts, this was said assuming the brushed motor is around 60% efficient. This is a figure I used back in my brushed days. I never used high voltage setups so maybe perhaps in your case the efficiency is much greater? In other words final mechanical hp after electrical efficiency should be about 3/4 - 1hp.

FE is Fast Electric. Your boat will be considered a Fast Electric even at 30mph!

Ryan