Thanks for all the comments.

Ideally I'd indeed like to have an isolated engine which would make my
dc system completely separated from the hull. It might be easy to do
if you have the money to buy a new starter motor, alternator and
senders, but I'm currently not in that position.

I'm now thinking of the second best option. Obviously I'll track down
the ground leak and correct it first. But then, what to do with the
engine? Put a ground wire to the hull or not?

Currently there is no ground wire. That means that any ground leak
takes current through the seawater to my prop/shaft or engine
internals (via cooling water) and via the engine negative wire back to
the battery. It eats away my expensive parts and paint.

If I would ground the engine to the hull, any ground leak would follow
the steel of the hull to that extra ground wire and via the engine
negative back to the battery. Hereby I assume that the path via the
steel is more attractive than "outside" via the sea water. I'd say
this is a better situation than the current because no electrolysis
takes place.

I'll post my problem to rec.boats.cruising too, see what they think.

Joost