If you walk as far as you can down the beach away from the tender dock and past the food area there is a dock or pier . On the other side of that there is some ok snorkeling from the beach with a bit of coral and some fish.It's not the Great Barrier Reef. You have to walk down that end to get to the food anyway so why not bring your mask and snorkel off the ship?
Enjoy your trip.
My family has snorkeled several times on Princess Cay and had an enjoyable time. The best place is like the poster above mentioned, by the far pier. It all depends on how calm the water is too but I'd say bring your gear and give it a try.