Not all puppies/dogs know how to swim. It is not necessarily an innate trait from birth. What we did with our Maltese was to carry him into the pool about 6 feet from the steps, lower him slowly into the water and, when it was obvious that he knew to keep his head up and paddle, let him swim to someone standing at the top of the steppings and calling him. Repeat a couple of times and then do it once a week or so, moving further away from the steps each time, until you're fairly certain he can handle it and knows what he's doing. Our Maltese is not interested in getting into the pool voluntarily but I am at least somewhat confident that if he did fall/get in that he would know to swim to the steps to get out.