5 years on boat batteries is pretty good. I was able to get 7 years on average. I have a smart charger that desulferize batteries. I do that before the boating season. Off season I would give it a trickle charge once a month, put them on a 2X6 instead of concrete floor and top it off with distilled water.
If you use the boat mainly for starting, a marine starter battery will be appropriate. If you do a lot of overnights, a deep charge battery. If you have a day cruiser and raft a lot, no overnights, the dual purpose should fit the bill.
A marine starter battery can handle the rough pounding of a boat far better than an automobile battery.
I have had batteries for 10 years. I was pushing my luck.
