I saw quit a few of these strange jellyfish while snorkeling the southwestern side of Maho Bay. They were resting on the bottom in about ten feet of water. While most jelly fish swim around with their head up and tentacles down, the Cassiopea spends most of its time with its’ head down resting on the sea floor and with its tentacles extended upward, hence the name, upsidedown jellyfish.
The upsidedown jellyfish can give divers a mild sting, which can be very itchy. According to Wikipedia: “The stinging cells are excreted in a mucus; swimming over the jellyfish (especially using swim fins) may cause transparent, essentially invisible, sheets of this mucus to be lifted up into the water column, where they are then encountered by unsuspecting swimmers,” but being that these jellies were in fairly deep water, this shouldn’t be a problem for snorkelers observing them.