The OEM alternator has an internal regulator. Since the market for alternators is overwhelmingly automobiles (12V), it's almost certainly an automotive alternator with a single-stage (single fixed voltage) regulator. You could try getting the make/model off its name plate and looking it up online, and you'll probably find out what voltage it's set for. My guess is 14.2V or 14.4V. It's possible there's an adjustment screw on the regulator that'll let you change the voltage, you'll find this in the alternator's documentation.
It will be a single-stage regulator, not a 3-stage regulator. This means it's single-voltage. Not a big deal generally, because it isn't run enough to damage a battery through over-charging. However, you could elect to have the internal regulator replaced with an external 3-stage regulator (e.g., a Balmar or a Ample Power regulator), in which case you can program it to match whatever batteries you carry. Probably not worth the effort, though.
Finally, whatever batteries you end up with, make sure you look up their specific charging needs. For example, the batteries supplied on my boat are Universal Battery UB121100 AGMs. On another boat, I had Lifeline AGMs. The charging profiles for the two are *dramatically* different. Yet both are AGM batteries.
Jeff