On the point of the alternator, if your hydro yielded 400W then you are looking at around 30 amps for a 12V system, not that an auto alternator is at all suitable as others have mentioned.
Perhaps a small 110VAC generator fed to a switch-mode power supply (SMPS) at the battery shed, HZ could be matched reasonably well but is not critical and most SMPS cover a wide range of voltages. Small China built gen-sets abound, just find one that the motor has died on, take the piston out and drive through the motor shaft.
Allan