ric wrote:
Does your boat not have a temperature gauge?
A temperature gauge really only shows you when things are gradually  starting to change from normal.
Blown out core (freeze ) plug at a very illegal speed/ 5500+  rpm ; rapid complete loss of coolant, your engine is seized before the gauge responds ( that was a very, very  expensive learning experience for me some 30 years ago !).
Blocked manifold and riser on one side on a V engine; the rubber exhaust overheats, melts and maybe even catches fire before,  if ever in fact,  the engine starts to overheat as the engine still has water flow through the other exhaust so it doesn't overheat.
Rather than a tell tale yellow  output  out of an imitation male appendage ( I try to be polite !),  a flow sensor  and associated alarm in the feed to each manifold/ riser would immediately give an indication of flow problems. I've heard of people using flow sensors from domestic heating boilers to do that. Actually, from around 2003 VP fitted temperature sensors in the risers. I am not sure whether these sound an alarm if they get too hot, I suspect they only cause the motor to run in a safe, low RPM mode.  I stand to be corrected .