Carnival provides beach towels in your cabin you can take, but dont lose them or they charge you for them. I would not pack them.
I personally dont wear the flippers to snorkel, just the mask and tube, but then in Hawaii we did not make such a deal about snorkeling.
A week ago or two on Conquest, I saw a girl who had snorkeled the day before and all her toes were terribly cut up and she had to wear sandels the rest of the cruise. Maybe bring some bandages .. just in case your toes are tender like hers were and the flippers cut into them. .. or take off the flippers .. if you can tell you are that cut up about to bleed.. its not required to wear them.
Conquest 7/21/13, Miracle ALASKA 8/27/13, Miracle HAWAII 9/17/13, Celebrity Summit, 11/30/13, Celebrity Summit, 12/7/13, Celebrity Constellation, 2/7/14, NCL Epic 3/2/14, Allure 5/4/14, Allure 11/2/14, Quantum 12/8/14