IMO AGM's aren't the way to go in a marine environment. Like has been stated before, they have different charging requirements than a conventional battery. If you charge them at anything over 15.5 volts they're toast. From what I've seen they are more prone to early failure, too. I usually see at least 5 years from standard automotive batteries, but am only seeing 3-4 years for AGM's.
I'm sure there are other opinions, and probably those that have done more specific research, but for my money a good standard deep cycle does the trick.