Brenard,
Several times I have used an automotive windscreen washer pump, the type that uses gears and so is reversable NOT the centrifugal impeller type that is not reversable.
Connected to brass tubes in the hull sides, they can turn models up to 5 kg at a realistic rate. A simple forward/off/reverse control has always been sufficent.
If the prototype the model is based upon does not feature a bowthruster, people rarely spot the two holes. It would not be too hard to fit this system into a large transverse tube for a more scale like appearance.
More complex and probably expensive methods are available if you prefer. I will leave that subject to others with experience to comment upon.
GlynnGuest