Make a brushed DC motor move "free"
A suggestion, not a complete answer:

Assuming a normal two-winding DC motor, not connected to power, there will be some residual field in both stator and rotor iron. This will generate AC voltage in both coils, and cause eddy current losses in both pole pieces.

The AC voltage is easy to avoid: just ensure both windings are open circuit.

For the eddy currents, it might help to demangetise both rotor and stator iron, so there are no residual fields. This is done with a decaying alternating current.

From a boat DC supply you'd need some sort of H-bridge driver that supplies AC to the motor stator and rotor, starting at say 50 Hz and rising in frequency until the current is small.