Rays tend to lie on the bottom and cover up with sand. They're nearly impossible to see. When I took a SCUBA class, we were on a dive and the instructor motioned "stop - danger" so we watched as he roused the nearly invisible sting ray. This was a really bizarre accident, and my guess is he swam right up over it, and it startled.

As far as his dangerous activities with crocs and other critters, I think they looked far more dangerous than they really were. I think he knew exactly how they'd act, and how fast they'd move I think he really knew his abilities and limitations.