We've used AGM's on our boat (and in cars and trucks) for 11 years now. In fact, the house batts we installed 11 years ago are still working well. They're a pair of East Penn "Deka" g31's, model 8A31DT. Deka's are the batteries that West Marine sells, and they are sold under several other labels. We've had the same Optima starting-type AGM's in our diesel truck for almost ten years.
Before switching to AGM, we've had Gel's and flooded batteries. Gels, which require significantly different charging voltages, are very long lasting in deep-cycle (house) use, IF you have the right charging systems. They are not very good starting batteries, unless your need for starting CCA's is pretty low, as it might be with a very small diesel.
I like AGM's for several reasons: no maintenance, no leaking/corrosion, very low rate of self-discharge when not in use, no worries about freezing over the winter, and very long life if not misused or abused - so much so that they don't cost much more than flooded batteries over time. Seems to me they are perfect for a cruising boat.
There are dual-purpose AGM's with lots of AH capacity that are good for house banks, and starting AGM's that put out a lot more CCA per pound. We use small (g34) starting AGM's for the boat's diesel, truck, and cars. Make sure you know what you're choosing. Look for a local commercial battery supplier rather than a marine store - you should be able to get them for 2/3 of the marine price. Quality brands are Deka, Sears Die-hard Platinum (Oddysey), and Lifeline (too pricy for my taste, and if I understand correctly they seem to have a greater need for AGM-specialized - slightly lower - charging voltages).
