Oh, boy. Here's a story from Australia (complete with video) of a group of kayakers being harassed by a 5m great white shark. One kayaker was knocked into the water by the shark, but was able to scramble back into his boat. After circling the group for about ten minutes, the shark finally moved on.
This is the latest in a series of shark incidents in Australia; a shark killed a snorkeler in another part of Australia at the same time that this incident occurred.