There was a pin hole in the back (low end) of the exhaust manifold near the raw water injection elbow causing water to suck into the engine filling the 2 rear pistons. It was a bit hard to start for a couple of years but I didn't think too much. I found it when I decided to find why the engine wasn't starting the way it should. I repaired it and move the raw water feed tube further away as I had to replace it as It was wasted anyway.

The pin hole was caused by a piece of rubber impeller that was stuck sticking half way out of the end of the feed tube. It would spray water back to the engine the entire time it was running. No problem when it was running but when the engine was stopped it would spray just a small amount of salt water back and would sit till it evaporated. After a while it just rusted a small hole in the exhaust manifold

It never ends does it.