There are good brands of batteries and cheaper one. What I install in customers boats are Interstate marine batteries, and they are considered to be one of the best brands out there. Interstate batteries, as long as they are taken care of and have their water level checked once every 6 or 8 months (and topped off as needed) will last 3 or 4 years in the marine enviorment. (that's in Florida they last 3 or 4 years).
Now that's if you actually take care of them. If you make sure the water level is higher, if you make sure your keeping corrosion off of the terminals, 3 to 4 years is what you get.
If you use cheap brands of batteries. Batteries you get at Walmart, or the auto parts store brand name battery. Those will typically last 2 years at best, and that's if you take care of them. If you don't take care of them, I have seen batteries only go a season and they are done, they just will not take a charge.
They key difference between a marine battery and a car battery is the lead plates inside of the battery are structurally stronger, and the battery is designed to take a beating. Cars have suspensions to dampen the roads, boats do not, and those batteries feel every bump and smash of a wake in a boat, which would not happen in a car.
That's really about it. If you want a good battery, Interstate is what I use and recommend. Maintenance free batteries do not last as long as the batteries you do and can add water to. Adding water to batteries has an inherent danger to it if you are not used to working with batteries, as there is high strength sulfuric acid in the battery. But they do last longer in the long run.