Need some help, when trying to get up on plain motor will run fine for about 30 sec.and then go in to gard. mode and alarm sound. If I turn the key off and re-start the alarm is gone but will do the same thing. water /temp press ok, oil pres/temp ok, drive oil ok. the only code it gives is over speed(rev.) If I stay under 3000rpm when trying to get up it will run all day long. I can not find any sensor for the over speed other than in the ECM. the tach does not show above the max rpm and I have changed the prop hub.(It has no slip) Every thing is the same as last year (prop, drive, ect.)
Any thoughts?

Depending on the scanner you may or may not get the code for low water pressure. But - generally the scanner will provide you with the amount of water pressure. Check that again...

The Freeze frame snap shot say's overspeed at 745 rpm , that can't happen. W/P was good. Merc says it has to be the ECU.
Has anyone else ever seen this?

also it does not have the shift switch and the DDT shows it to be in gear at all times.

crank shaft position sensor. Replace it