When I was putting in flooring over a concrete slab, they warned about high humidity absorbing into it like a sponge. I can only imagine what magnitude more of a problem that would be for submerged concrete/stone.

From a laymans perspective, seems like that's to clear off debris / surface grime and grit. Adequate cleanup would need to happen ontop of the surface prep.

It however would do fuck all about the water that's soaked past the surface and much deeper into the stone. NFI how long that would take.

For perspective I've spray painted some things, laid some bricks, grouted' some outlets into walls and have no career or formal education in those fields.