When deciding on the size of the charger required, you should consider two things:
1. the type of batteries you have; and
2. the type of cruising you're going to be doing.
AGMs can take all the amps you can throw at them (and are very good at burning up undersized alternators without good regulators). If you intend to charge the batteries when at anchor with, e.g., a 120-volt generator/battery charger combination, then you'll want a large-capacity charger in order to keep the charging time reasonable.
For a bank of two 8-D batteries, a 45 or 55A charger would be too small for this application. A 75A or 90A Iota, or other large capacity charger, would be more appropriate, assuming you have the needed generator power to run them. BTW, as an example, a Honda 2000EU will just run a 75A Iota, but not the 90A.
If, by contrast, you intend to be charging batteries mostly at dockside or while underway with the engine running, then a smaller charger would do the trick.
If you're talking about Iotas, I'd be mighty tempted to go with a DLS75/IQ-4 rather than a smaller one, since the price difference isn't that much and the savings in fuel would pay for the difference in very short order.