Obviously, you want to look good since you'll be capturing every moment on Snapchat and Instagram. But remember, music festivals = sweaty people, mud, spilled drinks, and other ick. If it's not washable or disposable, don't bring it.
Plenty of things can go wrong on a cruise vacation. You could get seasick, or sleep through breakfast, or lose at blackjack, or puke over the rail, or god forbid be on a cruise ship that's lost power and drifting out to sea.