Take a look at every driftboat you see in the river. If we all had them registered, we'd have those big ugly number/letter stickers plastered on the bows.
All the driftboats now have a transom rating on them. It should be on the metal plate that says what the passanger maximum is. If the boat is 16 feet or larger, AND you put the motor on it, you have to have it registered. Adding the motor is key to the requirement.
If you're still *really* unsure, take the boat and the motor to a USCG station and they'll make it perfectly clear ( heh heh heh ).
I'd take a copy of those regs listed above, and if someone tried to tell you that you have to register it, I'd ask them to point out which line says it's required, because a driftboat is not required to be, period.
diana
Good fishing