I am an electrician and air con mechanic
and I can't believe that a flow or pressure switch is not standard on marine
generators! Plenty of reports on the net of people replacing raw water impellers very often.
I was on a boat the other day and the guy mentioned how important it was to never run the generator
when sailing the cat. This isn't rocket science, measure the water movement through the generator and have a time delay of how ever many seconds is required to prevent startup or short cycling.
I would prefer to use a flow sensor set just above the manufacturers minimum spec. That would also tell you the filter needs cleaning
Presumably manual reset, with a limit on number of generator starts per hour.