My very first open water ocean dive was in Hawaii. We went down, swam a few yards and the DM took us through a swim through and pointed up, directly over our heads were schooling reef sharks swimming in a circle in the lave tube. It was awsome. I have never been in a situation where I felt threatened but then, I don't go looking for trouble either. The largest shark that I have ever seen under water was in a blue hole in the Cay Sal banks. We were swimming over the edge of the drop off and this enormus shark came blasting past us over the edge and off into the blue. Probably a reef shark, but a big one. I did get a little bit of the heebie jeebies after that, wondering what was making that big thing run like it was scared of something...