I have a new to me boat with this engine.  When I get the boat on plain, after a few minutes, I get they sustained temperature alarm beep.  If I power down, the alarm quits and the temp gauge goes normal.  I can putz around for hours and everything is normal.  I've blown out the lines and made sure it has a good stream of water coming out.  I also re-primed the oil.   What's next?  Water pump, impeller, etc.?