I've had my (1981) boat for about 6 months, and the engine (which was a concern of mine, due to uncertain history) has run flawlessly until last week.
I took some friends out for the afternoon, motoring out of the marina and for about 30 minutes to find some wind. Shut down the engine and sailed for about an hour. Kids started getting cranky, so I pulled down the sails and started up the motor.
After idling in neutral for about 1 minute, I get a high temperature alarm, so I shut the engine down. Guage reads about 160F (it's raw water cooled, so normal temp is about 120F). Waited about 10 minutes, but temp guage doesn't drop, so I tentatively restart to get some cooling flow. Although water appears to be coming out the exhaust, the temp stays high and alarm still sounding.
So I shut it down again and start checking stuff. Oil level is good, raw water strainer is clean (but we're drifting through a heavy moss/algea/seaweed current line). I can't find any obvious problems, so I start it one more time, and this time, the temperature rapidly drops, and everything returns to normal. Motored back home without incident.
I've had it out once since then, with no issues.
Any idea what might have happened? My guess is that a got a piece of debris on the hull strainer, which eventually dropped off on its own. Planning to change out the impeller soon, just in case that was somehow related.
(Incidentally, is there some law of nature that says stuff only goes wrong when you've got guests on board?)