Heights -surely you mean depths. Looking up isn't scarey, is it?
I don't have a fear of water, but being over deep water. That's being scared of heights proper. Imagine falling off a cliff - a few seconds of fear, then SPLAT. Simples
But falling off a boat miles from land. Once you run out of steam to keep yourself afloat, then it's a long, long, slow drop down miles down until your lungs explode.
Or swimming out of my depth brings on a panic attack. I can swim for about a dozen lengths non stop in SWMBO's gym - it's only 4'deep. But put me in a normal pool and I'm all a gibber after 1 length.
But they are legitimate fears, not phobias.
FROGS now. Nasty little bleeders. With them eyes staring at you, and they can't move nice and smoothly. It's nothing, and then woosh, they're all over you.
*shudders*