help - where do I start

The short story is; I have a Rinker 310 with twin 5.0 mercruisers MPI and bravo III drives. After several hours of operation an alarm we off (oil OK, Temp OK) Could not get the alarm off but did not have much of a choice but to continue. 20 mins later port engine would not go above 3,000 RPM. Pulled both engines back to 2,000 RPM (Alarm continued). 30 mins later PORT stopped ? I noticed a drop in the amp gauge prior to it stopping. When port engine stopped ? alarm stopped.
Continued with stb engine only. 20 minutes later alarm begins to go off. Noticed a drop in amp gauge. 20 minutes later lost power in stb engine. Engine did not stop but would not push forward like the trans was gone. Turned off stb engine. Started generator for one hour. Then started stb engine, amp gauge read 9 volts, and was able to make it back home.
I've been told it could be a "ground issue" - and/or a bad cells in the battery issue. Both ideas souond suspect. Any one out there have any ideas?

Re: help - where do I start

Seems pretty obvious to me. Start with getting the fault codes pulled on the engines and find out what went wrong. Or you could just randomly start replacing stuff hoping it fixes it ......

Do you have an amp gauge or a voltmeter? I'm ***-u-ming a voltmeter since you say it read 9 volts (an amp gauge reads amps - they're not the same thing as volts).

Depending on how your batteries are wired, a bad battery could cause what you described. Hopefully it's something as simple as that, a combination of bad batteries and/or alternators.