Well, I know there's a bunch of different models, but our pack 'n' play mattress is entirely waterproof. Now while I would not submerge it, I have had good luck cleaning it with just dishsoap and water. On set in stains I will soak a washcloth in soapy water and put it on the stain sopping wet. Let it sit for a while and wipe off.
I have also successfully used a mild bleach solution on it when I had to (diaper blowout...).