I can't stress this enough -- bring your own!!
Here is what happened to us: I was fully intending to just rent snorkeling equipment on CC. For some reason, at the last minute, I went to Wal-Mart and picked up two new sets and brought them along with a couple of extra masks we had at home.
Due to the unexpected hurricane, we ended up not being able to even go to CC. We did go to Nassau and I booked the Blue Lagoon excursion because we all desperately wanted to go to the beach. Blue Lagoon had NO snorkeling rental and if we had not brought our own, my kids would have missed one of the most memorable and enoyable times of this trip -- although they didn't officially go on the snorkeling excursion portion of Blue Lagoon, they were able to paddle around the Lagoon and were so excited to see the colorful fish there.
I was SO GLAD that we had the masks and snorkles and when we go on the Magic next year, even though we are going to CC twice on this cruise (hopefully!), I'm STILL bringing our own masks!
Kris