They have webbed feet.

An excerpt from the article 33 facts about Platypus
The legs of the platypus are placed on the sides of the body, which is a reptilian feature of this animal, as mammals have their legs under the body. Another reptilian feature is that the platypus steps on its toe bones on land, so it does not damage its toe membrane. When it moves slowly, its torso touches the ground. It lifts its body when it is in a hurry.