ON EASTER Monday, atop the deck of a 963-foot cruise ship floating somewhere north of Cancún, Mexico, Macda Hailu and DeShun Gilchrist were planning their evening. On the agenda: dinner, a comedy show, then hitting the club. The couple was there to celebrate Mr. Gilchrist’s 26th birthday. “It’s my first cruise,” said Ms. Hailu, a 24-year-old auditor from Byron, Ga. “It’s a good time. Lots of things to do.” The trip had its surprises, however. “There’s a lot of little kids.