Banana in the tail pipe...!

How much water could you have sprayed up your tailpipes just washing your car? Plus they'd have to get through the muffler too. Sounds like something else to me... Maybe the cats are clogged up?

Park it on a steep uphill and let it idle....

I don't know if it is true, But in offroading they say not to start your engine if your tailpipe is underwater because it can suck water back up the tail pipe. maybe if it is you suck up enough to get your O2's wet. try pulling them and drying them off and check if your pipes are wet

I seriously doubt water made it as far as the cats and even if it did it wouldn't matter and wouldn't hurt anything. Water vapor is a part of engine exhaust, so much so that mufflers usually have weeper holes to allow water collecting in them to drain out. Similarly, O2 sensors are usually clocked in exhaust pipes such that they lie above the 3-9 o'clock plane to that water doesn't collect in them. Cats run quite hot and any water in them would have flashed to vapor very quickly.

Mere spray under the car wouldn't result in water in the cats. Splashing from the tires washes down the undercarriage pretty thoroughly and even deep puddles can submerge pipes...

Any chance you got water in the ignition system or other electrics such as the MAF or air filter?