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.