Beth, you are not mistaken. The announcement is made on the tram, there are signs along the walkway and if anyone still has their literature from the cruise, check out the backside of the
Castaway Cay map. Under Friendly Reminders it states that "due to our commitment to the environment, we ask that seashells, coral and other natural treasures be left undisturbed at Castaway Cay for all to enjoy". For whatever reason, I'm thinking that I heard it announced a number of times when we were on board.
Tammi