Re: Yanmar coolant light on
I had to replace my sensor unit on the mixing elbow area with similar symptoms, but a fresh water system. I would unplug the sensor unit/temperature sending unit, and see if it solves the problem upon start up, while the engine is still cool.
If you are concerned about overheating, feel how hot the mixing elbow and the exhaust water is with your hand while it is running. You might also check the engine temperature with an infared thermometer.
If the engine is overheating, how is the water flow out of the exhaust on the transom? Is it normal or impeded? If impeded, perhaps your system needs to be back flushed with fresh water. Check your impeller and water pump to see if the front end is functioning properly. You might also have a clogged raw water intake.