Don't look at the cruise lines as much as the ships themselves. The Independence Of The Seas has a lot to offer for active people, including the Flowrider and rock climbing wall, plus a full size basketball court.
One thing to note, perhaps, is that Norwegian will allow parents to sign a waiver to allow those 18-20 to consume beer and wine. Royal Caribbean does not do this on cruise from American ports.
The biggest question is whether Cozumel is important to you. I've always liked Cozumel although it is so commercialized now that I long for the good old days of 20 years ago, when it was a quaint little stop.