I am no electronics whizz, far from it!, but presumably the raw output is A C, and the voltage will vary, with the frequency, according to the speed at which the generator is running.
So having rectified the output, why not then wire in a regulator? A 7805 will give you a 5 volt output from your DC, which you can then use to power USB type devices.
If you want to charge a Lead Acid battery, a 7812 will control the out put to 12 volts. A 7815 would probably be a little high for a 12 volt battery, as a fully charged Lead Acid delivers 13.2 volts. (Although car alternators are 14 volt)
A 7812 would deliver the right voltage to charge ten NiCds, or NiMH, (which are 1.2V/cell) connected in series.
You might get away with using a 7805 to charge four NiCds or NiMh in series (4.8Volts), particularly if you regulate the water flow to ensure that the out put IS 4.8V.
Just a few thoughts
Howard