Hey if it makes you feel more secure than go for it, I have seen just as many blown up powerheads in my shop from overheating with a Water pressure gauage as with out.
In the shop those streams are called dummy tell tells, becasue water can be pissing out of it and still not be supplying enough water to the engine.
Also when I'm running a boat with a jack plate and I raise the plate high, I might galnce at the water pressure gauage but I always glance back at the Dummy tell tell no matter what the guage reads so whats the point of the guage.
But boats are realtive everyone likes different things if having one makes you feel more secure about not over heating your engine then by all means install one.