Mike.
The same thing just happened to me with my Nautilaus battery, just going on 3 years. Mine is a deep cycle. My starting battery is fine, and it was stored right beside it in my garage. Although not directly heated, it was fairly comfortable in the garage.
I think I misused mine over the years in that I would charge it up too often. I would charge it every couple of outings, even though the reserve capacity was quite high. I read the other day that re-charging when it's at mid reserver capacity or highe is not good for the deep cycles - just like Irish said.
So, I need a new one to for this upcoming weekend. I'm not sure what brand I'll try this time. I'll have to check out some stores this week.