When I was a child I had a fear of swimming. I felt that it was physically impossible for me to stay afloat in any large body of water - that struggling, sinking, and drowning were inevitable.
This fear might have stemmed from having two close encounters with such a fate - once due to the undercurrents of Long Island beaches, the second time due to an oddly shaped pool. Sinking seemed to want me.
After many years of trying and failing, at the age of thirteen, I finally learned how to swim. But it wasn't in the ocean or in a pool - I learned how to overcome my fear in a river. It just took a little change in order for me to swim.