Battery Question

Hi
Would like to ask for help on battery charging.

On left rear side of boat there is two batteries.
The two plus cables hooked together and the two negative hooked together.
Which batteries should I hook the charger to? Plus and negative to just one battery or plus on one and negative on the other?

ideally I would disconnect one set of cables and charge the batteries individually. connecting the cables to the neg and pos of one battery will charge both, and you can even place the pos on one battery and the neg on the other battery, an both would charge. I personally would charge them individually.

http://www.marineengine.com/products/accessory.php?in=2901702
i would recommend this product for batteries connected in parallel which you have....if you dont have a device like this then one day you are going to go to use the boat and both batteries will be run down...this can be caused by many things but a partial short in one battery is normally the culprit... i speak from learning this the hard way many years ago
with a houseboat that was 12 miles by water from the landing..

you will accomplish two thing with this...you can charge both batteries with one charging device...both from an external charger and the charging system on your engine...the other is you can keep your start battery isolated and not drain from it with some other device installed on the boat..i personally like my ignition battery as hot as i can keep it while using the boat...