Most shark attacks are to swimmers on the surface rather than divers, it's reckoned because under the water a diver moving smoothly looks like another big predator and the sharks will usually keep clear unless they are already in a feeding frenzy. I've often seen sharks nearby but never seen any serious threat behaviour under the water. Snorkellers, surfers & swimmers on the surface look like an injured fish and so attract the sharks' attention.
Sandbender, I've swum with the basking sharks on Tiree, it's magical.