The pouch opens backwards and is closed by a strong sphincter around the opening, aided by an oily secretion from the surrounding skin that makes the seal water-resistant. The young inside can survive the short submersions their mother makes while foraging, and the arrangement is unique among marsupials.
Males have a version of the same structure. The male water opossum also has an abdominal pouch, into which the scrotum is withdrawn while swimming; the usual explanation is protection from cold water and from snagging in vegetation, though this has been argued rather than tested.
The rest of the animal is built to match. The hind feet are fully webbed and provide propulsion, the forefeet are unwebbed with long slender digits used to feel for prey under stones, and the fur is short, dense and water-repellent. Vision underwater is poor, and the animal appears to hunt mainly by touch, which is why it favours clear streams where prey can be cornered against the bottom.