A problem comes up if you leave from one US port and return to a different US port without visiting a “distant foreign port.”
If you leave from a port, disembark, then take a cruise back to the same port, that wouldn’t be a problem. So, you can take a cruise from San Diego to Vancouver, then back from Vancouver to San Diego. But you may not, for example, leave from San Diego, sail to Vancouver, sail to Alaska, and then disembark in Seattle. (
DCL doesn’t have cruises that start from Vancouver and end in Seattle, so this is just hypothetical.)
Puerto Rico is a partial exception, so you can cruise from, say, Port Canaveral to Puerto Rico. But you can’t leave from Port Canaveral, cruise to Puerto Rico, and then cruise on the same ship to disembark in Galveston. (Again, that’s a theoretical itinerary — I haven’t looked at actual routes.)
The Panama Canal cruises stopped in Colombia, which counted as “distant,” so they could go from Florida to California or vice versa.