2013 Merc 90 optimax

Help......boat in limp mode with intermittent beep. Manual shows cooling or oil problem. Oil tank is full and flushed motor several times and is spitting water. Decided to change spark plugs fuel filter, air filter cleaned manual lift pump and tested high and low fuel pump. Cleaned injectors still same thing. Rough idle won’t go over 2500rpm and beep still there. I think might be bad sensor or something. Any ideas? Can a bad battery cause this? Boat currently at shop to get plugged up to computer but they are two weeks out.

Sorry meant to post on boats and motors

JSouther, I have a similar issue. I have that same beep just turning the key on. While running the same beep persist. I couldn't get it on plane without it sounding as if it was gonna die, so I idled back to the ramp, turned it off and tried to crank it to load back on the trailer but, it wouldn't start. Just kept turning over but never cranked. I noticed water in my oil reservoir so I drained it and replaced with new oil. Not sure how water was in there. Mechanic basically told me it was time for a new boat. 93 Tracker with a 40hp mercury. I'm the only owner she knows so she's kinda sentimental to me! Good luck and be safe!