We rented a Cabana on Nellie's and snorkeled right from there. We saw many different fish and coral on the small reef there. We do have our own equipment, because even on excursions I wouldn't put a rental snorkel or mask on that somebody else used.
I've brought my own equipment to Labadee a couple of times and just gone off the beach. As others have said, there isn't much there. If your itinerary includes Roatan, Honduras I highly recommend finding a snorkeling excursion there. I've had several excellent experiences there.
We were told that there was snorkeling equipment rental at Adrenaline Beach, but when we went to check (approx. 1 1/2 hours after arrival) we were told they were completely out. We were able to buy equipment there at $80 each but gave it a miss.
If you have the equipment and it's what you want to do, then you'll enjoy it! The snorkeling view off of the beach in Labadee is WAY better than the snorkeling view from my computer desk. Just manage those expectations