PSA: Do NOT ATTEMPT CLEANING TECHNIQUE WITH CAT!
It depends on the cat. One of mine, Scotty, managed to turn himself into a bloody mess after eating a rather large, pregnant mouse, and so it was bathtime - it was like a blind baby eating a furry little chocolate cake.
A shower curtain would have been quite inadequate to contain a panicking cat. Fortunately, the other shower had one of those pre-fab "stalls" with glass on all sides, and a door. He may have been suspicious when I made him eat the mouse in the shower, as he's seen running water in there.
Yes, it was bathtime.
I did what Ned did - got in with him. Scotty was nervous.
The the water turned on. Letmeoutletmeoutletmeoutletmeoutlet!!!!
His response was interesting though - he'd go nuts and try to paw his way through the door if he was sitting on his own, but if I held him under the water, he just hung there and took it, without so much a struggle, meow, or even a grumble.
Sorry, no poop though.