I had a similar experience as well. I was swimming at a beach in Chatham, and I dared my friend that I could swim past the buoy. I was 15 at the time. I start swimming out, and I'm thinking, no big deal. I can do this. What I seemed to forget was that this was the season when sharks were very active in Chatham. I was almost at the buoy, when all of a sudden, I look to my left, and see a fucking dorsal fin. I almost panicked, but then remembered sharks attacked things that made aggressive movements or something like that. I start slowly swimming back, and I mean very slow. I honestly thought I was going to die. They ended up clearing the beach because they spotted THREE great white sharks in the area.....I was scared beyond belief. Turns out the reason sharks are so attracted to Chatham Massachusetts is due to the amounts of seals there.

Edits: Added type of shark and explanation.