AllerSoft Waterproof Mattress Protectors do the job
I bought 3 AllerSoft Fitted Hypoallergenic & Waterproof Mattress Protectors. Before putting them my new mattresses, I tested by pouring 1/4 cup of water on the fabric and no water soaked through. I was also happy that these arrived with absolutely no chemical smell (critical for products intended for allergy sufferers). I liked them so much, I bought 3 more protectors to have a backup for each bed. They wash and dry well.