I agree that the email doesn't really say you can hunt from a power boat other then when the motor is off and the boat has ceased is forward motion... It would be a poor defense...
You have been told by a federal game officer that he would charge people for doing it... If the conviction went through then suffer the consequences.
But i agree, it has gone from common practice, to "I would fight it", to "you should be asking somebody of authority".
How much more do you want, that email is just some cut and paste of the regs, you have been told straight up by a Federal wildlife officer (the branch that write and enforce the federal regs!) that it is not allowed and is a chargeable offense.