This swimmer by the name of Scott Fairchild was practicing breath holds off a coast in San Diego, when an unexpected visitor showed up. That visitor? A Great White Shark. He calmly swims away once people let him know what's going on.
I'm sure we'd all swim away if we were in Scott's shoes, but I don't know if we'd all be as calm as Scott was. With my luck, I'd be praying the theory of "punch a shark in the nose to deter it" actually works!