How Deep Does A Phobia Go?

And that's no pun intended because I'm talking about my fear of deep water. I can't watch movies that take place underwater, I can't watch people swim in deep water, and apparently I can't look at the oceans on Google maps using the satellite feature. I get the creeps every time. I tried zooming in on Lake Baikal in Russia...scared the bejeezes out of me - I had to close the window. I wonder where this phobia came from...