While I would agree that wet cells are the most economical solution long term if you are NOT living aboard and at anchor much of the time....decent AGM's are actually more economical long term if you ARE living aboard for several reasons:
1. # of life cycles of recharging is several times greater than wet cells
2 the SPEED of recharging since you you can recharge at 100% of capacity rather tha 20% of capacity saves $$ in diesel since you can recharge fully in far less time.
Before I make comments let me say that Cam has more practical experience with AGM's than I do. So, though I disagree somewhat on his point #1, it is with respect that I do so.
#1 - I have not seen any data that shows AGM's last longer than wet cells, in fact, I have seen data showing that GOOD wet cells can outlast AGM batteries by up to 400%. AGM's do have a better lifetime than golf cart deep cycle wet cells but only by about 20%. This assumes properly maintained wet cells which is really big assumption, because wet cells are easier to kill if you don't take care of them (i.e. check water, etc). I have seen HYPE that says AGM's last longer than wet cells, and it sounds a whole lot like the same hype that said gel cells were going to take over the entire battery world.
#2 - Cam is right, AGM's can take a full on charge and wet cells can't, and that does matter when charging from a generator or alternator. That isn't an issue though when using wind and solar for most of your power generation, and it's less of an issue with a large battery bank. Depending on how you are charging batteries I think this could be a deciding factor.
Do NOT mix battery types...go all AGM if you are a full time cruiser or all wet cell if you are not (and don't need the maintenance free aspects). You may vary battery capacities as long as the type doesn't change.
I don't think it's as simple as wet if the boat is sitting and AGM's for cruising. You can get more bang for the buck with wet. I agree if you have limited space (we all do) and you can only fit 8 batteries, or 10, or whatever, and you want the best bank you can get regardless of the money, then yes, buy the AGM. But if you are like me where space isn't the overall limiting factor and money does actually influence your decisions, then you can have a battery bank 2-3 times BIGGER if you buy wet over AGM, and that's not something to be taken lightly (it certainly will be hard to take it "lightly" when you have to carry all those freaking batteries LOL). Say that again, if you have 1000$us to spend, you can get 2 - 3 times the number of amp hours with wet cells as you can with AGM. With the cost of lead going sky high for wet cells, maybe AGM's will eventually make that less of an issue.