No doubt this action is discouraged. We contacted Ketchikan CBP and they were very understanding, stating that this was not uncommon. Notifying us of the possible $300 fine and suggesting we contact the head of Passenger Services once onboard to inform them and complete any necessary paperwork.
I've printed out the Passenger Vessel Services Act they've cited from Wikipedia and it clearly states a fine of $300, but also says "in order to embark in a US port and desembark in a second US port, the cessel must visit a port outside of the USA". Since our first port will be Victoria, B.C., shouldn't this exempt us from this act?