I actually winterize his boat every year for the past few years. This is kind of a long story but here we go. Toward the end of boating season he noticed his boat was just not running quite right, but couldn't quite figured out what is was. He checked all the fluids and for whatever reason installed a thermostat before taking it a local shop. He stated there was no water in the oil but this shop called and said there is water in the oil. He didn't put the boat in the water after checking the oil so he knows the oil was good. But upon picking his boat up he decided to take it out. Shortly after taking off his boat started to over heat. He shut it down and made it back to shore and trailered it. He then took it to a different shop and they pressure tested the exhaust manifold (and I assume everything else) and said the manifolds was causing water to get in the oil. They also said the manifolds cracked due to over heating. I just don't get how manifolds could cause water in the oil.
