No worries about Trunk Bay changing too much. The land it is on was donated to the U.S. Park Service by the Rockefellers under the condition it would be preserved as a U.S. National Park.
For the snorkel gear don't get the expensive or the cheapest. I have had good luck with the low end US Divers brand for tubes and fins, but I have a mid-range mask. For the one time thing you can get it all at Wally-World in one kit. You can also rent it there, but I soon found out I can have my own for near the same price. If you want to rent a snorkel vest, I think is $5. If you take a Ship tour they usually supply the equipment. But I take my own regardless.
I usually put my stuff by the Lifeguard's stand and say Hi to them. No one will bother it, and these Guys mean business if someone walks on the reef!
Of course you are going to do a good job if you have the best job in the world down there. A U.S. Government job as a Lifeguard
Best to you,