Originally Posted by Russell Clifton
Might try this. Turn off your 12v battery switch so all of the battery voltage is turned off on the boat. Check to see if you still have the 2.7v. If it is gone, turn battery switch back on, start turning off 12v breakers, checking for the 2.7v after each one until you find the one that is supplying the 2.7v Now you should know which system is supplying the voltage. Not exactly sure what to do from this point. Good luck.
If when the batteries are totally disconnected you still read voltage you have a stray current issue.
Put one lead of you meter on neg connector other in sump water, if you read voltage, its stray current, check bonding system and zincs