I thought I'd add a few comments to this post, it seems to be a pretty popular subject. Once again, I'm coming from the perspective of autos, not having a boat yet.
First, I'd highly advise against powering anything from one battery in a pack. Even with a low drain (10MA) you will constantly bring that battery out of balance. The longer between charges the more you imbalance the pack. And if you need to discharge the pack fairly deep, even once, for any reason (emergency?), you will have a much higher chance of ruining a battery. The old saying is "Most batteries don't die, they are murdered." It's much better to use a DC-DC powered by the whole pack. This gives you power for all your 12 volt needs (or whatever voltage you need) and respects your pack. You can get some pretty high (and low) efficiency DC-DCs.
When looking at multiple chargers, remember that you now multiply the points of failure. Taking it to the extreme, some people use one small charger per battery. This can work fine, each battery gets full attention of it's charger. But now you multiply the chances of something failing. What happens when one fails or a charger gets unplugged by mistake? Will you notice it? As you motor out you could be reversing a battery. You need to monitor your pack very well.
But there are some simple (and cheap) techniques for protecting your pack. One of the best (simple and inexpensive) is the Lee Hart regulator. In the auto EV world, Lee is a well respected voice. Basically the regulator is a simple device that will bypass some of the charging current on the "fuller" batteries so the weaker one can come up to charge. This helps ensure that all batteries in your pack get a full charge. It doesn't address all problems, but it is a pretty effective basic protection for standard 12v lead acid batteries. The parts are inexpensive and anyone with a soldering iron can make them.
Here's a link;
http://www.seattleeva.org/wiki/Zener-lamp_regulator
Dave Cover
http://www.evalbum.com/2149
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