I have never been to a snorkeling-approved beach that didn't have a ton of vendors hawking rentals. You should have no problems finding rental gear. The boat will occasionally have snorkels/fins for sale for just as cheap as you would rent them in port, but they are usually pretty poor quality (speaking from hearsay, haven't worn a Carnival snorkel myself - any input from others?).
As an avid swimmer, I'm going to politely make a note here: Renting snorkel gear will usually cost between $10 & $20 per person for the day. My wife & I bought the following kit a few years ago:
It has lasted us through 3 cruises, numerous East coast beach adventures, swimming expeditions through the Florida Keys, and two levels of SCUBA certification. The gear is rugged, easy to pack in your suitcase and even includes a nice bag you can stash towels / cards / money in when you debark. Even if you only snorkel a couple times on one cruise, it pays for itself. The $40 each we spent has been one of the best investments we've ever made and I HIGHLY recommend giving it a shot. You don't need a fancy snorkel or expensive Cressi fins to have an enjoyable experience.