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There are three different types of batteries that get used in boating - marine starting battery, deep cycle battery and automotive style battery. An automotive battery is a poor choice for use in a boat. A battery gets rougher treatment in a bouncing boat than it does in a car. Auto batteries are made with lighter materials and thinner plates. They will work in a boat but they won't hold up.
The two remaining types, starting and deep cycle, have quite a bit of difference between them. A cranking battery pumps a lot of electricity for a short period of time. Then the battery quickly recharges. The internal design of the battery is made to support these high bursts and rapid recharging and will give maximum life when the battery is treated in this manner. This is what's needed for starting an engine.
A deep cycle battery can give a continuous flow of electricity until it is fully discharged. Then you can recharge the battery and do it all over again. The design of this battery allows the discharge - recharge cycle to happen over and over without destroying the thing. You use this battery for trolling motors, radios and other accessories.
Now, if you use one type as if it were the other you are going to damage the battery. End result is shorter battery life and less dependability. A starting battery that is deeply discharged and then recharged actually causes some of the electricity producing material to come loose from the plates. Because of this the battery may only last a fourth as long as a deep cycle battery doing the same job.
If you use a deep cycle battery to start a motor, you are asking it to deliver a heavier dose of electricity than it was designed to. When this happens the electricity-producing ability of the battery goes down a little bit. The next time you use the battery to start the engine it has a little less power and again its ability to make electricity goes down a little and this continues until you get a battery that is weak.
At this point a lot of people like to complain about how cheap batteries are made and demand warranty. Most battery companies will give the warranty, and increase the price of new batteries to make up for it. One way or another you pay.
The best way to keep batteries is to use two, one for starting the engine and one for everything else. Many people use only one battery. If you use this method keep in mind that the life of the battery will be shortened. If you have just a few accessories I would recommend getting a large starting battery. Keep regular maintenance on it.
Another way to go is to find a combination type battery. It acts as both a starting battery and as a deep cycle. I don't have much first hand information about how these hold up but the manufactures say they work as great as anything you have ever seen (and I've never heard them say otherwise).
If you do use two or more batteries and you seriously depend on them I suggest you invest in a battery isolator system. The isolator will charge each battery independently of the other(s) and keep any one battery from overcharging. You'll maximize the battery life and power capabilities. If you spend a lot on batteries that go bad before their time this might be a money saver. It could also be cheap insurance that you will have a good battery to get back home with.
If you have an outboard that does not regulate the alternator output do not use a maintenance free battery. Unregulated systems will over charge a battery (I have seen as high as 16 volts). Maintenance free batteries don't like this. You will boil out water and some of those batteries don't have a way to add it back. A battery that is low on water will self destruct. Check them on a regular basis.
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