When Virgin Voyages announced last year that the cruise line’s first ship, to debut in 2020, would be for adults only, it surprised many in the industry. Cruises are typically marketed to families and boomers. Recent years have seen cruise ships add wild theme park-like attractions onboard, ranging from multi-story in-suite slides to go-kart tracks to indoor skydiving. Families are big business in the industry. Marketing the no-kids move as “Adult by Design,” Virgin Voyages, a joint venture between Richard Branson’s Virgin Group and private investment firm Bain Capital, looked to fill a void in the market. Virgin Voyages CEO...