I can't speak specifically to NCL's procedures, but I recently needed to disembark a Royal Caribbean cruise a few days early, in Civitavecchia on short notice. I had to leave my passport with Guest Services the day prior to disembarkation, and the morning the ship docked in Civitavecchia, I had to be cleared through customs (the disembarkation officer handled - which is why they needed my passport). After they cleared my passport, the disembarkation officer found me, returned my passport to me, gave me an "in transit" pass, and personally escorted me and my luggage off the ship. Hope this helps ... obviously your situation is different than mine, but I don't think you can simply show up with your stuff and walk off, like you said.