Snorkeling in Fiji we came across a reef shark - actually he came over to see us. If it weren't for the fact that my brand new bride was with me and I felt a need to appear manly, I would have run across the surface of the water back to shore.
As it was we just slowly swam back to the boat.
The worst experience that I have ever had was when diving in Mexico I caught the heel strap of my left fin when I was back rolling out of the boat and into the water. As a result I rolled under the boat and was disoriented. When I tried to surface I hit my head on the bottom of the boat which left a smudge of blue on the side of my head. Then once I did surface and get my bearings, my fin slipped off.

Other than feeling like a real dip the rest of the dive was OK.