Mark,
The guys are right, not all batteries are alike. I live in Florida which is not good conditions for wet batteries. In cars, bikes & boats. Most problems folk here have is evaporation of battery fluid. The more you have to add water (use only distilled), the weaker the acid solution. Thus the battery weakens.
The AGM (sealed) batteries are what I use in all my of vehicles & they work great. My boat (2) Group 27's are 4+ years old have never been a problem. I use a digital charger, but I also use a 7 day timer for battery charging. I just don't trust having the charger on all the time. Every other day, I set the timer to run for half to an hour. I only have my bilge pump on auto, no other systems on.
I'm not crazy about D8 (golf) batteries either or any wet cell. The D8 size & weight I don't feel is worth it. But it might depend on systems running. I don't use an inverter or have A/C etc. running on my 30 footer. If yours is like mine, know how to charge your boat away from the dock, you can take extended trips & keep powered. It's simple.
You motor out, shut the engine & sail. Depending on the length under sail & systems running, you later start your motor, run for a half hour & your batteries are back up. I run my motor on approach to anchor at the end of the day. In the morning I run the engine while still anchored or leaving. The AGM's benefit is they re-charge quickly over wet cells. This was one claim that made me first buy them. They will not work if upside down. But if you find yourself in this situation, you have bigger problems with your boat.
There is a new dry cell battery out recently, but it's still in it's infancy the last time I checked were not available in many sizes. It claims to hold a charge for 7 1/2 years! I feel the jury's still out on this.
CR