While getting a bottom job done on my 1980 33', I happened to look closely at the intake for the raw water feed to my diesel. It is the stock one that is shaped scoop-like with long slots for the water to come in. Inside the housing, were several small mussel or oyster shells. They were too large to go into the hose, but too large to come back out the intake slots - I guess they got there when the boat was sitting for a year or so before I bought her - crushed them and got them out - seems as if this could cause a "mystery" overheating problem if one of the shells blocks the intake hole or is partially sucked into the hose to the engine.