Many of my customers are of the opinion that marine batteries have a very limited lifespan.
Most people feel lucky to get two seasons out of a set of batteries.
They mention that after a couple of years on the job, efficiency wanes, and it's a rapid down-hill trip from there. You may find it hard to believe, but your vessel's batteries should last well over the two year mark. If they are quality batteries and have been taken care of properly, a five year plus lifetime is not unusual. This condition results in the battery's plates drying out and also causes a deterioration of the positive plate grids. Usually due to a poor engineering and cheap materials.