I concur with Rob on his advice. The thermostat is often the culprit - especially on boats that sit for long periods in salt water. On engines that do not get flushed, barnacles have a tendency to grow inside the housing around the thermostat. Here's a picture of barnacles growing inside a thermostat housing cover. The barnacles can impede the opening of the thermostat, thus causing the engine to run hotter that it should. "Salt never sleeps"