I've seen plenty of sharks while diving, and interacted with a few (a strong bang on the nose with the butt of my dive knife turned an overly inquisitive blacktip reef shark into a formerly inquisitive shark at Martinique's Diamond Rock). However, I was most apprehensive while surrounded by a large school of barracuda in the Bahamas. I didn't even know they schooled! As I was snorkeling at the time, and they were circling me rather closely, my pucker factor was very high. In the next moment, as barracuda are wont to do, they had disappeared, leaving me wondering if they had even been there at all!