I have yet to have the high temp or low oil pressure alarm go off for real, but they are a great reminder to turn the ignition off
I also have a CO alarm. I think these are a necessity for a boat where you will have people sleeping with the engine running.
Alarms I am going to add when I get a chance and spare $$$$:
1. Coolant flow
2. Exhaust temp
3. Low fuel pressure (maybe)