You can get "1-wire" alternators relatively cheap. I picked up a 200 amp one for just under $100. Included the internal regulator. Just connect to battery and provide +12V to the reg.
If you are just charging a 12V batt, that is all you need.
And IIRC, when Chrysler was the first to start putting alternators on cars, they made quite a big deal about how it would keep the battery charged even idling at night with the lights on. Probably moot in this case, as you can change the drive ratio to get whatecer speed you need with the water flow you have. -- One man's Magic is another man's Engineering.
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