As shrew stated, batteries are killed by their owners a long time before they "wear out" by use. What I mean by that is the owners treat them poorly by discharging below the 50% level (usually because they just don't know their batteries state of charge), or they regularly operate the batteries in what is termed a partial state of charge (PSOC), not fully recharging them before using them again.
If you recharge your batteries using a generator or by moving the boat (with the engine) you are for sure not getting to 100% charge. It would take at least 6 hours of generator run time to achieve the necessary 100% charge (that is an approximation, but is in the right ballpark).
AGM's have some advantages, but you pay for them. If you are not going to "treat them" well, then you are spending a lot of money for little gain. If you operate at PSOC regularly, then buy flooded lead acid deep cycle golf cart batteries. They take abuse as well or better than most other types and are the least cost per amp hour.
Installing solar panels will help with the PSOC, but if you just go dock to dock, then maybe AGM would be good??