On our catalina 34mkll I recently had an overheating problem - not huge, but enough to cause concern. I replaced the impeller without any improvement. We had to return from an outer island on a wet day with no wind and so we motored gently for 10 hours or so without a major problem. To add to the concern, when we got back we discovered the bilges were almost full to the top with freshwater! I didn't see any relationship until I replaced the thermostat and twigged that as the engine was heating the hot water tank hotter than usual it was releasing the pressure valve on the tank and filling the bilges with hot water and then refilling itself from the freshwater tanks! That was a relief as it had been a real mystery. I think the thermostat needed replacing anyway, but the real culprit turned out to be the seedpod of a mangrove which had got sucked into the cooling water inlet (which we had inspected) and then slowly worked its way up till it partly jammed the inlet to the water strainer basket!! No problems at all now.