Just as important as your batteries are the quality of your battery chargers.
I've used Guest charges for years, and they've been great.
What I do is to turn on the chargers about once every 3 days to top off the batteries. Then I turn them off. I don't leave the chargers on all the time.
A bad/cheap charger may overcharge and cook a battery. I had that happen one time, using a cheap charger. The battery got incredibly hot. It fried it.
The absolute worst thing you can do is to leave your batteries discharged, especially after fishing and draining them way down.
You can buy a brand new battery, fish with it, then let it sit in the cold for 3 months (discharged), and it will be totally ruined. If a battery is neglected, then it doesn't matter how new it is.
There is absolutely nothing more important on a boat than the batteries.
They are the lifeblood. Without them, you ain't going nowhere, and you ain't doing nothing.
I used to buy Gel batteries, they are expensive, but I've gotten 10 years out of a couple of them.
Now, I'm going to do just like Rickie.
Get them from Wallmart. $100 bucks for their biggest marine battery, with a 2-year warranty.
If you're diligent about making sure your batteries have the proper amount of distilled water (if applicable), and you keep them charged regularly, you will get the most out of them.
You don't have to use a battery in order for it to discharge. It will discharge just by sitting.