If you have your own and plan to snorkel many times while on your cruise, do bring it along. They just soak the snorkel in salt water between uses so it may not be as sanitary as your own. If you don't have a snorkel vest, you may have to rent one where required.
If you have your own stuff and have room to carry it DO IT!~ I got one of the worst colds of my life using snorkeling equipment on the very first cruise we took that we snorkeled on - a mask is optional - take your own snorkel if you can. Masks and fins don't go into orifices in your body like a snorkel does.
If you do bring your own make sure that it is clearly marked with some type of personal identification so it doesn't get lost in the shuffle or you don't have to fight with a tour guide to keep it. Most of the stuff they supply on the ship and on tours is solid black, so if you can find colorful stuff you should be fine.