In the case og the Yanmar 2GM20, I like the idea of a temp snap switch mounted on the engine. It could be wired in parallel with the water switch which would turn on the alarm and light... I can put my hand on the engine block after a hard run so the normnal engine temp on the outside with water is under 130F. a 130 to 150F snap switch would allow for earlier detection..
Cal 9.2 #19 SilverSwan