I just bought this boat and have had it in the water about a dozen times, and it has ran great up until today. It has a bone stock 2000 502mag mpi in a 27' fountain fever. 150 fresh water hours. Bravo 1, 1.5 ratio outdrive. Full engine and outdrive service done before I bought the boat. Today I ran the boat hard at WOT for about 1 mile against a 27 baja (he lost). About 1 hour later the alarms go off. I stopped the boat and checked things out. Restarted the engine, everthing is ok. Take off again and the alarm goes off at about 2700 rpm. All fluid levels are good. Drive oil was a little low(not much) and I added oil. Oil pressure is 55, volts,14, water pressure is 9, at 2500 rpm, Water pressure is 35 at WOT, 4900 rpm, oil temp is 140. Water temp is (strange). Sometimes it will float up and down from 130 to 160,170. Engine is not hot. I can put my hands on exhaust or the thermostadt housing and it is cool. The water temp gauge only does this somestimes. I disconnected both of the sending units on the thermostadt housing, the gauge went dead and the alarm still went off. Stopped the boat and hooked both sending units back up. I drove the boat about 3 miles at 2500 rpm with no problems. Bumped it up to 27,2800 rpm, alarm went off in about 30 seconds. I turned the engine off , restarted the engine and let it idle, everything was fine. I left it in neutral and bumped the throttle up until it got to 3000 rpm and ran it like that for about 2 minutes, everything was fine. Let the engine idle and put it back in gear and took off. Ran it at 2500 rpm, everything was fine. Bumped it up to 27,2800 rpm, alarm went off.
I am very mechanical, I have built many auto engines and build harley davidson engines on a regular basis and also very good with eletronics but boat engines and outdrives are new to me.

HELP !!!!!!!!!!!!!!!!! Call me 434-660-1957

Could be a knock sensor gone bad. On a 500EFI it doesn't become "active" until ~ 3000 rpm. When it tries to pull input from the sensor, if its bad, it will set off the alarm but it can be a 2 beep "soft alarm". Is it a constant alarm or a 2 beep intermittent alarm?

not constant alarm, It is a 2 beep, about every 10 / 15 seconds.

I can run the boat at WOT even with the alarm going off.

I am going back in the morning and disconnect the drive oil sensor and see what happens.
Might disconnect the water pressure sensor if I can figure out where it is.
I will do this 1 step at a time.

I got home this evening and started reading post on this websight.
Lot of good information here !!!!!!

Drive oil sensor and water temperature sensor will give constant non-stop alarm. I'm pretty sure is no water pressure sensor on 502 MPI engines - only on new PCM controlled engines. IAC's (idle air controller) will also give 2 beep alarms (on 6.2's, 496's, etc), but the engines typically die at idle when they are bad (had that happen 3 times on a pair of 6.2 MX engines).

Are you sure it's not the depth sounder alarm?