My first boat was a a 1971 32 ft. Islander sloop, that I owned from 2002-2007. Soon after I bought her, I replaced the automobile-quality starting batteries with rugged marine-grade AGM 105ah Group (31) batteries. In my experience, that was a good decision. I never regretted the extra expense for the AGM batteries, because it gave me peace-of-mind - and eliminated a lot of potential maintenance headaches.
Given my client workload, I haven't had much time to do any sailing in the last year. So, it was just two weeks ago that I finally decided to get around to replacing the old wet-cell Group 31 batteries that came with my current boat (a 1963 35 ft. Pearson Alberg, that I purchased last year).
Another deciding factor on choosing AGM batteries: Due to the very limited and exceedingly tight and difficult access to the batteries, it would be exceedingly difficult to maintain wet-cell batteries.
Since I had not kept up with topping the old wet-cells with distilled water - there was also the small matter of their untimely demise. As in, dead, like a parrot.
So, off to West Marine I went.
Three AGM 105ah Group (31) batteries ending up costing me $779.97 ($259.99 each).
Extracting the three wet-cell batteries turned into an all-afternoon (and painful) endeavor - and it was only after I began trying to wrangle an AGM into the most difficult of spots - that I realized; "damn, the new AGM battery is 3/4 of an inch longer than the old wet-cell".
After 6 hours, and pulling the muscles in my back - I accepted that it wasn't going to happen that day -and decided to give it a rest for a few days or so.
Now, two weeks later, my back has recovered, and I'm ready to try and tackle the job again this weekend.
Worst-case scenario, if I cannot find a way to squeeze that one battery into the tight spot [it happens to be wired as the starting battery] - I may have to consider relocating it to somewhere else on the boat and rewiring to a new location.
Oh, and the battery terminals on the AGM batteries are slightly smaller than the previous wet-cells apparently - so there is another issue to be sorted with not being able to tighten the battery terminals completely, for at least one set of battery cables.