Yes, when you book a snorkel excersion through the cruiseline, you do get the mask, fins, snorkel, and vest if you want one. I always use one, many times you forget the direction you're going and end up "out there", its great to have the vest and be able to rest. Some you can adjust how much air you want in them.
You can get the snorkel, mask and fins for around $50 at your local sprots store, thats what we did and it has paid off. I'd say if you plan on cruising or snorkeling again in the future, you would be best just to buy the equipment, happy sailing.