I finally gave up on the concept of a battery that will actually last in the marine environment and resorted to fraud to solve the problem. It is my understanding the the fundamental problem with marine battery life is that they simply don't get used enough.
In Canada, we have an auto parts store that sells their own brand of batteries. The marine batteries typically lasted about 2 - 3 seasons tops, and the so-called warranty perhaps contributed $10 towards a new one when it was pro-rated.
With the help of one of their sales people, I discovered that if I bought their top of the line automotive battery, I could pay a measly $9.00 extra for full replacement warranty for up to 10 years, regardless of the number of times I replace it! The only wrinkle is that the warranty isn't valid if the battery is used in a boat.
So, my warranty is registered as a truck battery (the old Chev pick-up that lives at my cottage and is used for for runs to the dump... and which is totally non-existent). Since this is an automotive battery, I needed to add marine terminals to it, and I need to remember to remove these before I return the faulty battery.
I am pleased to say that I have returned these batteries on two separate occasions now and received shiny new ones, free of charge, each time.
These are starting batteries, but I am not concerned about the deep cycle aspect as I don't tend to sit still long and when I do, I'm generally only powering a VHF. (And I have two batteries, so always keep them isolated when "parked"...)
So even the best batteries are still shyte when it comes to boats, but this little "work-around" seems to work wonders.