MetalJunkie I used to have that same problem but since I had it as a teenager I was closer to the source of the fear time wise, enough to work it out rationally and find out why I'm afraid of looking horizontally while under water (I always wear goggles when swimming). Although in my case seeing it on T.V would give me a chilling excitement -like watching a horror movie- instead of the discomfort you experience. But while swimming it's be really discomfort and worry.
Initially the only reason I could come up with is that I'm slow and crippled under water and would never be a physical match for a creature who's natural habitat is water, so if that creature was of an aggressive nature I would be done for, especially -and that's where the phobia comes in- if I can't see horizontally past a few meters and then it's plain blue. That plain blue scared the fuck out of me even if I was just in a big swimming pool and there's no chance of anything attacking me there.
That line of thinking took me to movies and that's where I easily arrived at the problem; I saw the JAWS movies at a very young age, we had absolutely no monitoring or the idea of checking movie ratings and parental control advisories at our household at the time, much like all of the homes in my country at the time, it just did not occur to anyone.
My dad used to go to work trips to the states and he's always return with a dozen VHS tapes of his favorite movies/new movies in general (movies that would otherwise unavailable for us cause there was no such thing as cable at the time) and these movies were viewed openly in my presence, a period that ranged from me being 5 years old up to 10 years old.
Clear blue means the murderous shark is gonna show up from within the blue. Once I realized that the under water view didn't bother me anymore.
The only other thing that's similar -in relation to discovering phobia sources to be something you watched on tv as a child -is a recurring dream that rarely left me since childhood of me lying on a rail road track and my movement is limited to how I can center myself between the metal railings so the train would pass over me without killing me. That comes from watching The Good, The Bad & The Ugly when you're 5