How to get from 3 phase variable high voltage AC to 230V AC
If your generator is permanent magnet, as I suspect based on the varying voltage, the raw power coming from this will not only vary in voltage, but it will vary in frequency too as the water velocity changes. You have two choices here;


Vary the water flow so that you achieve a constant speed relating to
50Hz, then take whatever that resulting voltage is and use a
transformer to step it up or down to the desired 220V. This is by
the way difficult to achieve with precision. 
Take the variable voltage and frequency into a rectifier to make
DC, then use an inverter to recreate AC from that.