L.P.
Yes, they are referring to the vinyl pants used over cloth diapers and you can find them in Target or Walmart. They are usually called Training Pants. I don't know if the non-disposable swim diapers count.
We swim at the Herndon Community Center where they have the same regulations. No one has ever checked but the plastic pants are an added measure to ensure a bowel movement doesn't get released in the pool. I believe they would have to close the pool if that happened.