Fear of deep dark water

After almost 30 years of swimming in rivers, lakes and in the sea, I just noticed I've got this fear of deep dark water.
I was diving at Curacao when I suddenly got far away from the coral reef and all I could see below me was that huge dark gap. I just panic and started to swim back, so desperate I almost drowned!
That fear was completely irrational. I never had a phobia before.
A couple of years later I tried again in a less deep beach, at Venezuela. The result was the same. My heart started to beat really fast and I couldn't bear that situation any longer.
It's not fear of the water or any animal, neither of heights. I just feel there is 'something' pulling me into the deep.
I don´t know if this phobia has its name or what can I do to get my fear of deep dark water cured.