What model Yanmar do you have? Larry is correct that all of the smaller
Yanmars commonly used in yachts have a warning system - both low oil
pressure and high water temperature. The Yanmar panel has warning lights
and a warning buzzer.
In addition, many Yanmars have several places where you can add a sensor
into the water system. You can have both the buzzer and an analog temp
gauge. I added gauges so I can watch the temp go up as it warms up and
even if the fridge goes on. I get quick feedback when a belt breaks,
since the buzzer doesn't trip until about 190, but the normal temp is
about 170. The only downside is that my wife has decided that 180 is
"too hot" and she starts to throttle back when I'd rather be running
closer to the design RPM.