Searched and did not find anything about this. Getting a block water pressure error on 04 MPI 5.0 Motor. New to us boat so sorting things out. Replaced water inlet hose, cleaned T-stat housing, replaced/inspected impeller housing and flow tested the pump, blasting water out of the hose that goes to the T-stat housing when idleing in water, well in spec. Scan Tool showed the error, removed and cleaned it again, now the Low Pressure error is intermittent. where it was solid before cleaning. I would seem the sensor is simply faulty. Scanned some post on other forums and there are references to AC-Delco Oil Pressure sensor that is what Merc is using with their numbers on it. It has a pretty simple job, report pressure with a resistance change within a specified range and survive in water.
This is a pic of the generic PN fo this, not going to use anything but GM Delco part, just using this so you can see that the form factor is the same.
ACDelco 213-1543 GM 12574309

I do not want to use a lower quality or higher fail rate sensor but not wanting to pay Merc $160 for a part that if is the same an be had around the corner for 50 bucks. When you get the beep code you don't know what it is for so have to find a scan tool or swap in a known good part for the suspect part. Either way at 50 bucks knowing those are high fail items you could carry a tested spare.

Is there a thread on this water pressure sensor I did not find?
Two questions, I don't have access to the scan tool any longer.
1, If I replace the sensor and fault is not active will I still get the 2 beeps indicating that I HAD a fault or will it stop when the sensor is in range?
2. Could I simply bypass this with a resistor and make it think the pressure is fine or does it test at startup for a ramp or range or some other nonsense? I am totally comfortable using a water pressure gauge and temp to determine if I have water flow. I do not want a future failure of the sender to force me to hunt the scan tool down to see what it is beeping about. The whole thing seems stupid, when it alarm and if it were working you would likely have a hot engine or gauge would show low. Merc just made things more complicated than it had to be by adding an expense sensor. I mean, why not have a "you ran aground sensor", " you spilled your beer" or "your dragging your drive up the ramp sensor"?

Also, scan tool showed the water temp at 158 fast idle in water after 5 minutes, the gauge showed 180. Risers stone cold. The temp sensor for gauge is at T-stat housing, is the ECU reading the same sensor or somewhere else? Betting gauge on helm is just off. I suppose I could figure it out with IR gun or disconnecting the water temp sensor at gooseneck and seeing if the scan tool immediately reported a fault or change. Just curious.... OK so that was 3 questions

Change to “survive in unfiltered/unpurified water that is subject to pieces of: sand/mud/corrosion/organisms/rubber/seaweed/trash……….”

This is not in a more suitable and friendly environment of constantly filtered oil in a closed loop system

Imho: Change the sensor , then go from there if you have to.

Understand the environment, just do not SEE any changes to AC Delco part from the Merc part and the Merc part seems to need cleaning and still fails a lot. So question stands, does Merc just take an off-the-shelf part from "somebody" and assign their PN or was it specifically engineered to a Merc spec? I get sticking with OEM will yield a known result but perhaps for $130 less you can still get the same result.

Update-
AC DELCO ACD# 213-1543 GM# 12574309 OIL PRESSURE SENSOR
Installed about 4 outings ago. Scan tool shows normal pressures and no alarms. I did have to use an NPT 3/8 to 1/4 bushing. It is possible but I have not done the research that there is an updated part number. It is simply a sensor that reads oil pressure on many GM engine applications. I will update next year if I think about it or ASAP if the GM part fails. If nothing else it could be a carry spare at a low cost.