If it's too powerful, then elctronic PWM is one way to go, but you could always change the propellor for a smaller diameter or a finer pitch one. PWM and marine are not good bedfellows!
Essentially the thruster motor is a DC starter motor and will easily pull hundreds of amps. In a yacht there is often a load battery next to the motor, long cable runs cause voltage drop when trying to pull 200A. If you have a battery next to the motor, try disconnecting the battery - the long cable run from the engine battery will act as a ballast resistor. Beware - the cables will get hot! If this is too much, then a delayed switching in of the load battery could work. When you press the 'thrust' switch, the motor can start on the long cable then a relay/solenoid can switch in the load battery after - say 2 seconds to save overheating the cables.
Tim.