Try your local NAPA store. They have several listed with various pressure limits. I just bought a 1-1/2 lbs. switch for my electric fuel pump for $8.00 For water pressure you can plumb an oil switch into a Tee off of the drain plug where the pressure should read 3-5 lbs. at idle

By gollly this system works. Ask a question and someone can give at least a swag answer. (that's Scientific Wild Ass Guess) for those that don't know! I like how F31 thinks.
I had an overheating problem that was kicking my @)#*&%^@)#*(%^)@.
Intermittantly the right motor would overheat at cruise and above. Let it idle and everything was OK.
Come to find out, when I would flush the motor out using, the hose adapters, the shuttle valve in the H2O line would hang-up and partially block the inflow of H2O to the seawater pump.
I would check flow by removing the header cooling hose while running. It showed flow, at idle but could not keep up with WOT PRESSURE. If I would have had F31 theory in place it would have helped troubleshoot.
F31........let me know what pressures work for system operation. Because I'm going to put in a similar sys. ......so I can watch the Bikinis.....or lack of.