You will get the equipment if you sign up for a snorkeling excursion through the ship. If you just go to the beach on your own, you might be able to rent some there. In either case, you don't know whose mouth was on the snorkel last.
I bring my own but don't bother with bringing the fins. They are not needed and only add a bunch to the load. In fact, water shoes are nice to have when walking on the rocks.
We have our own masks and snorkels that we bring along everytime. We got them at Walmart for under $20 and they are very nice sets. I don't use "used" snorkels, for me it right along the line of wearing someone else's underwear. If you just want to bring your own snorkel along you can get those at Walmart for about $1/each so you know the snorkel is sanitary.
I have my own mask and snorkelwhich I always bring because of comfort and the quality of my own. (Which, by the way, was USED when I bought it, AFTER renting for a couple of days on Oahu from Snorkel Bob's.) My husband has fins as well, but those things are SO HEAVY to carry around, that I skip those. I just rent them if we are on our own. Exursions, as mentioned above, include the free use of equip. Want to mention something though. For YEARS I used the regular rental equip. offered on snorkel tours. Also rented stuff from beach rental places on various islands & in FL. Hasn't anyone ever noticed the bleach water/soapy water (usually in a big plastic garbage can or in buckets) that they ALWAYS put your stuff into on an excursion? You generally drop it in yourself once you have finished w/it. I have NEVER, EVER gotten sick from using used equip. Neither has my husband, our neighbors or our kids. I guess you could live in fear that somewhere, at sometime, someone might not use proper sanitizing methods. Also, you generally do not catch a cold from DRY germs. So if the equip. has had time to dry out afer even just being rinsed off w/water, you shouldn't catch anything, either.