Going on a luxury cruise feels like a big of a hangover from the 1950s and 1960s. They're an incredible way to see the world, and they generally offer as incredible an experience in the 21st century as they did in the 20th - but don't expect to see too many young adults on one if you decide to book a cruise yourself. They haven't engaged with them as vacation options at all.
Time is precious to millennials, and they don't have much of it to themselves. In the same time it would take them to get from one country to another on a cruise, they could visit five or more destinations by plane - and thanks to budget flights they can do it for less than half the price. Cruises will always have an audience for as long as retired people exist - but as the retirement prospects of millennials don't look great, even that may stop happening eventually.