If you have never snorkeled, then I would either rent from the ship, if they offer the service, or just use the excursion operators stuff. I don't know the cost of renting on the boat, most of the tours the equipment is included.
After our first cruise, we decided that we liked snorkeling, and planned to do it every cruise, so we bought our own. The biggest advantage is that you get a mask that fits, and a snorkel that only has your germs on it. The disadvantage is that it is just one more thing to pack. RCCL boats have a dive shop on board, so you can get fitted and purchase there if you wish, or visit a local dive shop in your area.