The diver probably has some food on him, which the stingray is trying to get.
I visited Stingray City in Grand Cayman a lot of years back. Part of the tour package was that they gave you small squid to feed to the stingrays, and they would climb up you, out of the water for that snack. Also, there were a lot of stingrays in the area. We were instructed to shuffle our feet as we walked, to avoid stepping on one. The swimmer in the picture only needed to hang out for a bit before one or more stingrays would have come over, looking for any handouts.

That said, the experience of Stingray City was absolutely worth it. Between that, and snorkeling at the barrier reef, I have a lot of fond memories of my time at Grand Cayman.