As with all cruises leaving US ports, they have to have at least one stop outside the US before returning to a US port. For the Hawaii cruise it was a one day stop in Ensenada. Note that the cruise ship in the picture below was not ours.
I can't say there was a lot to do in Ensenada. We'd already been to the main tourist attraction, La Bufadora, which has a water spout that is hit or miss depending on the tides and waves. La Bufadora also requires you to walk a gauntlet of vendors selling food and trinkets. So instead we just strolled around some of the nearby shops. If you're into wine or tequila there's a lot to buy, but not much in the way of beer, unfortunately.