Also you can ruin them if you charge them with over 14.2V and some alternators go to 15V.
That's one of the reasons a charger regulator (as Minnewaska says) is a must.
I think you're thinking of gelled batteries, not AGMs. In fact, AGMs can and should take a higher absorption voltage...14.6-14.8 VDC and, the faster they can be charged the better (i.e., the largest charging source you can fit on your boat).
As was noted above, AGMs need to regularly be fully charged
or they will sulfate and suffer a shortened service lifetime.
This is hard to do on a mooring, unless you have adequate solar power and a proper charge controller.
I'd not choose AGMs for a boat on a mooring UNLESS it had a sizeable solar charging system.
Also, it's true that if the batteries are significantly discharged they may place a very heavy load on your undersized alternator and, with no protection in the way of temp sensors, burn it out. This is because AGMs can take an enormous amount of amperage, will demand it from the alternator, and alternators typically fitted on boats are not capable of putting out high power for a long time.
Battery chargers will not likely be burned out by attaching them to AGMs, but they'll take a long time to charge. This is no problem if at dockside for a long time. If your charger doesn't have an AGM setting, use the flooded setting. Flooded batteries and AGMs have very similar charging profiles.