Many years ago local fishermen stopped in this area to clean their daily catch. The unwanted fish parts were thrown into the water which attracted Stingrays to the area. As the fishermen would arrive the Stingrays would gather waiting for their free meal. The Stingrays began to associate the sound of a boat engine with food. Eventually it was realized that the stingrays could be fed by hand. The area became a tourist attraction and is now the most popular tourist attraction in the Cayman Islands.
It is not dangerous if you are calm and listen to your guide. Most cases of people being stung by stingrays occur when someone accidentally steps on one in shallow water. Stingrays are defensive and are not intentionally aggressive.
I took the same trip eleven years ago...it is an experience that I would recommend.