My wife and I took some local advice and hiked around to the rocky-beach side of Leinster bay, where you can enter the water and swim about 100 yards offshore to snorkel along Waterlemon Cay. This was the best snorkeling we experienced in St John from a beach area(go by boat to get out to some even better spots), because the swim out and depth deters more casual wading swimmers from standing up on the reef. Thus the reef is very much alive and less damaged than the coral around some of the bays that are more popular for casual swimmers and Cruise Ship excursions, such as Trunk Bay. There is a great variety of marine life here, and taking you time is rewarded as you pass around the small island
This leads me to the only warning advice I can give: you really need to go extra heavy on the suntan lotion and/or wear a swimshirt. I normally never burn and have olive skin, but because the sights here are so fantastic, I forgot to reapply and had no swim shirt, and the sights kept my bare back to the sun for over 150 minutes!! Suffice to say, the burn was pretty severe...
Sun protection measures aside, this is a highly reccomended half-day, and the swimming around the sandy part of Leinster bay is nice too.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.