Nope, it's not just a joke about scaring the s--t out of their meal before taking a bite. Sharks do circle when they spy something in the water, but it's not necessarily because they're hungry.
Sharks have less than stellar eyesight -- they often rely on their ability to detect the electric fields of hidden prey -- and are forced to take extra time to fully comprehend what they are seeing. To understand and study what they spy floating in the water -- surfer, seal or otherwise -- sharks circle to get a 360 degree picture, and more often than not they get it wrong.
When it comes to attacks on divers, sharks release them after the initial strike 67 percent of the time. This high "catch and release" percentage is thought to be due to a misinterpretation of their vision. After their initial attack on a human, sharks discover that it's not the blubbery seal they were hoping for. When it comes to going after definite prey, sharks usually attack from below rather than skimming the surface in a circular pattern.
While Hollywood will mostly likely continue to make circling sharks and impending attacks synonymous, rest assured (?) that you're not necessarily shark bait if you see a fin doing a 360.