What Lisa and I are wondering is if there are other people who have this--and if so, what it's called. I started to do some research online, and I found references to it, but (1) nothing helpful, like a name--more like other people mentioning the same phobia, and (2) their descriptions of what they were afraid of were making me gag. (Lisa, skip to the next paragraph; I'm about to give examples.) Huge tree trunks under water. The underwater workings of flume rides. Huge boulders under water. Seriously: gagging and feeling horrified/hot/sick.
For me, it is basically a fear of Large Things that are Under Water. (You can see why I thought Lisa's name for it was so apt.) One of the worst things I ever saw (sorry, Lisa, it's more examples) was the cover of a Jaws paperback: the swimmer balanced on the surface of the water, the huge shark coming up from underneath, and so much water still below the shark. I also look away if a movie shows a submarine coming up to the surface: all the water running off the sides FREAKS ME OUT.