Electrical – How to move objects within given distance using DC motor precisely

In addition to the cornucopia of methods that burst forth from Russell's answer you could consider to add yet another to that rich bounty- if you have sufficient off time in your maximum PWM duty cycle you may be able to harness the back EMF of the motor itself as a tachometer signal. This has the advantage of being sensorless. It can be a suitable method for motors that are permanent magnet or that have a separate field winding. There must be sufficient time for the energy stored in the rotor inductance to die down, after which you can simply sample the voltage with an ADC.

A detailed description of this method can be found here. The back EMF is proportional to the RPM of the motor, and of the same polarity as the applied voltage (assuming a PM type or fixed field voltage) when measured at the terminals.

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.