The sting rays in Grand Cayman are accustomed to being around people and see us only as a source of a quick snack....that being what we feed them. Sting rays do not have teeth. Their mouths are just suckers, hence the "hickey" caution. If anyone gets bitten, it was probably a fish that got greedy and tried to sneak in for a bit of free food. Living in Florida, I have a lot of respect and caution about all wild things, in the water and out. I do the sting ray shuffle when I go swimming. But, I loved the tour in Grand Cayman.
Along the same lines, if you do a dolphin encounter or swim with the dolphins, you are dealing with trained animals used to people. Dolphins in the wild can be dangerous. Usually not, but all wild critters can be unpredictable, even me.