here I believe there is at least 10 / 3 beeps faults for the HP 525 and YES sea water pressure failure is one of them - the 3 beeps for sure. You can have a bad sensor or shorted or a real water pressure / volume issue. Start with changing to a new sensor or least cleaning up ur old one. and make sure all connections are good to it.

Bryan for the 525 IAC is 5 beeps or least should be thru merc. FYI

Do you need me to list some of the 3 beep sensor faults ?

Well if the water pressure code came back , check what the sensor reads with out running and running to see what the sensor reads. Just replaced one that read 0 psi with 15 psi manually applied.

Dealer got a list of them. They thought for sure it would be the block pressure sensor since that code came up historically and again after they cleared the codes but they thought that by unplugging it, the beeping should have stopped?

Dealer went through and individually disconnected each sensor on that motor and alarm still sounds. In reading through Mercury's 525EFI manual, it looks like most of the 3 beep alarms have 15 second intervals? Mine is longer between alarms but not always the same. Sometimes it will be a minute between and others it may be 2 minutes. Just when you think problem solve, 3 beeps!! Maybe a computer glitch?

Switch PCMs between motors. That would eliminate or point toward wiring harness.

With the software it really should not be this much of an issue. Run the engine and monitor the sensor outputs. The sensor and or wiring to it will stand out. The information from the problem circuit would be skewed, too high too low or just fixed (computer puts a fixed value so it can continue to operate). Unplugging the sensors would activate a different code as the PCM would not see even the sensor rather than a sensor out of range.

You can also the software's data logging (log some new data) and go back and look for an intermittent issue.

I had the exact same thing happen to my 600sci last weekend and it was the water pressure sender on top of the oil cooler, it was plugged with hard water so Nordic replace it but I think it would still work by just cleaning it out they said it is very common. It is very easy to get at to check its on the left side on to of the oil cooler on the back of the engine, the new one was $129.

Remember 525 EFI is completely digital, not analog like a 502 MPI, it's a warning indicator when does it come on, those of us race 525 efi's know that the first indicator of 3 beeps is when you're at the rev limiter of 5400 RPM (stock trim), this will happen with a 24 pitch under normal condition and a 26 pitch prop with some trim. What is your running temperature, if you're running digital gauge, running temperature should stay under 160 degrees, if you sucked up some weeds while running check your filtration box and clean it if necessary, oil pressure will also sound off if it's a digital gauge, also watch for carbon build-up in your throttle yoke, this will cause a delay in dropping the engine back down to an idle from cruising speed, this will also cause an alarm. When are you located and I'll tell you were to take it??? I know all the codes on a 525 EFI, mine was built from a 502 to a 635 and produces almost 900 HP (N.A.) 1200 HP under 3.5 boost if I wanted it. The 860 HP with 920 FT:LB is enough for a 28 foot boat. They're are really under-powered for single engine application (just a 502 MPI on steroids).