[SkyMarshal]

Even "wall of water" is misleading, as that's not what it is at all. A horizontal wall perhaps, but not a vertical one like in the finale of Abyss.

One way I've used to explain it is to imagine you're on a boat at sea and a tsunami passes under you. You wouldn't even notice.

The boat would slowly rise a few feet and stay there for a few minutes as this huge mass of water passed under it, then slowly drop back down to its original height, and no one would be the wiser.

It's not until the mass of water hits the shore and keeps piling on for several minutes, that people can see what it is.