Re: Charger Sizing... (too big?)
You can't have a battery charger which is "too big". The batteries will take what they're going to take in the way of amperage and no more, provided that the charging voltage is correct.
AGMs, of course, can take a LOT of charging current...at least as much as their AH rating. You're not likely to be able to fit a charger which can provide all that amperage :-)
At dockside, of course, it really doesn't matter very much. You just need a charger sufficient to counteract self-discharge of the batteries, and handle expected loads. But, when you're cruising, charging capacity is important unless you plan to run a generator for very long periods of time.
Which battery charger to fit now depends on your plans and your pocketbook. The best current charger out there is the Sterling AKA ProMariner Ultra Pro series, which come in several sizes up to 60 amps. These can be combined if you need extra charging capacity. But...they're pretty expensive. Figure $500 or more for the largest model.
The most economical good charger out there is the Iota/DLS series with the IQ-4 smart-charge option. These come in sizes from 15A to 90A and provide a lot of charging capacity for the money. While they're not "marine rated", they are used on many boats (including my own and many of my client's) and do a very good job. A good source for these is Northern Arizona Wind and Sun.
There are other good chargers on the market, too, including Victron's (top-notch but expensive), Mastervolts (ditto), and others.