Hi, tjones27
Er, you don't need a resistor
Your arithmetic is impeccable, but not applied in a good cause
first, the S400 7.2V is
not a voltage, it is the size of wire it is wound with, and hint about a voltage to start at!
You can run this motor at any voltage from 1.5 to 20 so long as you keep the current below 10A, and there is no way that a bow thruster is going to exceed this
and it says that at max efficiency it drains 3.3A (I run a S400 7.2 on 18V in a ducted fan aircraft - it has lasted about 10 flights so far)
If you are running the bow thruster with a speed control - presumably reversible, then staying away from full throttle is (effectively) the same as reducing the voltage applied.
So do, by all means run it off the main battery
If you need tactile reassurance take the boat out from the dock, do a minute of bow-thrust pirouettes and feel the motor
It should feel hot but not scary hot
You could easily (if needed) wind an aluminium or copper cooling coil round the motor and cool it that way - but it shouldn't be necessary.
btw - resistors of the size you calculated do very commonly exist - they are called lengths of resistance wire salvaged out of dead hair-dryers (I have 2 daughters) and they are also presented to look like car light bulbs! 12V, 12W is one ohm!
andrew