Why does Kiki flood the floor of her flat to clean it?
She is scrubbing the floor.  It is a very common way to clean floors -- a search engine turned up many images showing people doing just this for &quot;scrub the floor.&quot; Given that the water is on it only in the short term, the damage is not extensive even over many years.
It can leak, like any other water spill, if the floor is not watertight.