Dan,
FLJ is right, as always.
But it might be worth knowing that the power difference might not be as great as the current differnce (or it could be more)
Reason: the prop in the BT tube. I confess to not knowing the blade section or twist but most marine props - like aircraft props - are mainly one-directional. If you spin them t'other way at the same INPUT power you will get much less water pumped
For a bow thruster the terms forward and reverse have no real meanings, but connecting your ESc so that "forward" drove the bow thruster prop in the "backward" direction and VV. you might find you had reasonably equal thrusts in both yaw directions.
Specialist bow-thruster props are completely symmetrical and reversible (have a look at Vetus BTs
http://www.frenchmarine.com/ProductsC.aspx?CID=17 )
I would have thought that probably you only need a little reversible ESC for a bow thruster. I believe that Action do things like this
andrew
ps after writing the above I have had a look at some model bowthrusters and see that the graupner one I found - the 1785 - is S400 powered and uses a paddle, not a propellor, and the Marx one has a symmetrical prop
Apologies for a not-very-relevant post, left in case it helps anyone making one of their own