Sometimes an old ground cable will go bad by
corrosion inside, even if it looks good on the outside it will cause poor grounding and poor starter performance.
Yanmar starters never
work hard because Yanmars usually start by just bumping the key, they don't usually go bad after only 300 hours.
Was wondering if your engine is equipped with an anti syphon valve, which prevents
water from syphoning in, filling your
exhaust with water and going into the
head filling the open valved cylinders with water. Hopefully, this is not the case. You need to see if you can fully turn the engine over by hand, if you can't, than maybe that is what happened. I don't trust anti syphon valves and always turn my
raw water intake off,leaving the key on the handle so I don't forget it is turned off.