Evidently, this High Priestess knows something we don't. Perhaps it's that knowledge that is allowing her to glide the way she does, at once of this world and not of it. I don't think that even her toe is touching the water, but her silky robe is, indicating that that which clothes her is of the mundane world, while she herself is not.
I'm looking at the water where it reaches the sky, and it's just become apparent to me that apart from her two pillars, there is no land, nothing tethering her to the earth. Water is so... I can't think of the word I want here, but it's like... it has no shape of its own, it just fills whatever space it is given - perhaps that's the way of the HP's knowledge. It has no shape of its own, no boundaries, just fills her, molding itself into the very corners and edges of her being.
And perhaps what we don't see, water spilling off the edges of the picture - there's no shore for it to lap against, certainly - is like her knowledge, filling what it can and spilling over in its abundance.
I am not saying this well lol but it makes a lot of sense in my head!
Edit - after looking closely, I've noticed that at the tops of the pillars, there are little carved figures, that look a little like the Fool, or at least like the head on the wand he's juggling in the Gilded Fool card. Does anyone think this means anything? I might not know much about Tarot, but I know enough that images in cards are never accidental!