its a ball of water ?

Yep!
Well, frozen water, anyway- my guess is that the cats owner- okay, person who the cats graciously allow to care for them- had a really large bowl, filled it with water, froze it, and then dumped it on the tray so that the kitties could play with it.
Unfortunately, the ice melted a bit, so poor kitty stepped in freezing cold water with no warning!