I've never heard of a water pressure alarm. The Merc's have an overheat alarm, but by then it can sometimes be late (just prior to engine blowing). I (and a lot of guys) like to have both a pressure gauge and a temp gauge on my motors. This is in addition to the overheat alarm. Sort of like a belt and suspenders. You have a lot better indication of whats going on before you have the problem.
Rich