By some accounts a great white shark is one of the three main species where, if seen, should be considered cause to end the dive. I believe the other two are Tigers and, I think the worst of the three, Bull sharks. Of course this statement would be and probably is subject to debate :)
It seems that there is a growing seal population in the area off the eastern shore of Monomoy Island, a National Wildlife Refuge off the southern elbow of Cape Cod, Massachusetts and this is what is attracting the sharks. These sharks by staying close to the otherwise protected seals are natures control of the seal population. A good thing.