Hahahaha, my cat Dropshadow does that, he just loves to try to "catch" the water, then he'll paddle furiously!
I believe he's mystified that water can be solid, wet, yet be "invisible"....or maybe he's just crazy!
You can't get him to stop, either, so I've come to the conclusion that he earnestly believes he's "saving us" from something. It's the only way of explaining this obsession of his.
So when a guest would attempt to use the bathroom, and yell,
"AAAK! What on earth do you DO in here?!?"
I'd just explain that Dropshadow had saved us again.