I have the same thing. When I was about 12 I went to Bull Shoals reservoir in Arkansas with my aunt and uncle to visit his family. We swam in the reservoir, and it was a blast, the water was very warm, and exceptionally clear, and also very very deep. We swam out to the buoys ringing the swimming area and my uncle's mother and I went from buoy to buoy.
Probably a good 150 feet from shore, at the farthest buoy, I decided to dive a bit, having brought my snorkel and mask with me. I went down about 10-12 feet, following the huge chain down into the depths, when what seemed fairly close below me I saw a shape move in the water. It was a dark shadow and it was followed by other shadows, and it seemed to be fairly large.
It more than freaked me out, it set off a full-on Jaws panic attack. I turned and swam for the surface as fast as I could. At the buoy I put my face in the water to look below me and maybe 10 feet down were 6-8 very large fish, at my age then probably near to my length in size. They seemed to be circling the chain and moving upward. I freaked and tried to splash away, not swimming very well in the process, yelling for my uncle's mother to come back and help me. She told me to just hold still and she would come help me (I wasn't a very good swimmer to begin with, never have been).
I did, treading water, and then I felt something brush along my leg. It seemed like it never ended. Another something bumped my leg and the final straw was when something took my foot in its mouth and chewed on it a bit. I kicked at it and it moved away. I was in a full-on freak out, and just then my uncle's mother got to me. She looked in the water and said "they won't hurt you, they are just carp", huge freaking carp.
From then on I have been seriously freaked out about very deep water.
