I am loosing my mind. I tend to be very good at tracking this kind of stuff down being in the 12 volt industry for 10 years. I can test draw on the batteries with the battery switch in the off position and I get zero draw unless the bilge pump comes on. I have the Mercathode fuses open currently. I had the boat running three weeks ago and came up the next weekend to find dead batteries. I had a heck of a time getting them to take a charge. I finally got them to charge up using some one elses charger so I figured my charger was bad. I went down to the boat to run it this AM after using the boat all yesterday and it barely started. Ran it for a bit and it started up fine. I do not have my load tester to check the batteries (they are brand new) but does this sound like it could be a Mercathode issue? I want to leave the boat in the water but not at the risk of the batteries going down and the bilge pump not working!

could be a dead cell in the battery itself...yank off the covers and test each cell...

[FYI] The mercathode will draw any where from .5v up to 5.0 volts max depending on how bad the water is in your area till dead batteries. Also dose the batt, switch turn off the stereo 100% IE, the clock and memory that is good for a week at best some times, If I remember right that is a round .5 volt worth of power also. make sure the switch turns every thing at the dash off. the next test to do is unhook all the power wires to the batteries have it kind of dark under the engine hatch and start touching all the power wires one at a time a few times with a 5 seconds rest in between taps of the wires looking for sparks, The bigger the spark the more power you are loosing some place. then once you find a spark, And you think it is coming from the helm area then it's time to start unpluging wires or pulling fuse one at a time I would start with the stereo and unplug the wire form the back side. Also you will testing the main cable from the batterie switch to the batt, It's self with it off then with it on and looking for sparks. Also if their is a lot of wires hooked up to the switch then you will need do the same test to each wire at the back of the switch. If you find no spark at all then normaly the batteries are junk. If you have power at the dock you could install a batt, cahrger. Or get on of them solor power chargers. Good luck I hope this helps you out some.

a bad rectifier in the alternator will drain the battery over nite. disconnect the alt wiring and see if your voltage draw drops dramatically

If they had a hard time taking charge,,,DEAD CELL !!!

Would this still be the case wit the battery switches off? I also see no draw when I have the battery switch off. Could one bad battery draw down the other? I assume so but over night?

Yes,,this can happen over night ,,since the dead cell needs juice and the other cells want to help.

If your switch has filed wire to it then I believe But I'm not 100% sure of this, But that could be part of the draw. I say that because the filed wire will let you run the engine when the switch gets turned to off with out hurting the alt, And if the switch did not have the field wire then you would back feed the power to the alt, and burn it up if the switch went to off when the engine was running. That is how I understand it to work But I could be wrong But worth looking in to.

with my race car, ive had batts die over night, scratch my head, test for draw etc etc. and each time it was a bad cell.
put a new batt in and i was good. one time i put the car down from a wheelie hard next run, batt deal. it broke a cell inside, we cut it apart and found it. seems like a bad batt even tho its pretty new. allot of batts cant take much pain from wheelies, or waves, and it shocks them, brakes them etc.

Thats why i like the OPTIMA !!!!!!

+ they have a full warrantie of 1 year !