[tsubaki]

I know I'm not going to describe this correctly. At least FERM will straighten it out.
A stator may produce an over voltage due to the rpm's and the regulator is used to keep the voltage within a certain range during the rpm fluctuations.
A regulator may become defective or damaged for whatever reason and let the full voltage from the stator pass through or not any at all.
Now, why is the regulator damaged? That can be a booger to determine.
Is it possible the batteries are too large or weak and needing voltage constantly or might there be a heavy current draw from other things in the boat?