A closed-loop cruise is a cruise that departs and ends in the same U.S. port, for example, Fort Lauderdale to Fort Lauderdale, or round-trip to Alaska from Seattle.
Prospect Point in Stanley Park, Vancouver
These cruises have to meet certain criteria to qualify.  Every voyage on a ship that’s not U.S. flagged, departing from and returning to a U.S. port must, according to maritime law, visit one foreign port of call. This is why you’ll find Alaska cruises stopping in Vancouver, or Western Caribbean cruises calling in at Cancun.
To qualify as closed-loop cruises, the places these itineraries can include must be contiguous territories of the United States. That includes Mexico and Canada, because both share land borders with the U.S., and several of the islands of the Caribbean.