Not true!! All dogs can not swim! It is natural for a few dogs to somehow know how to swim, but not for most, so don't believe that for one minute. You are right to worry about them falling off the dock, because chances are it's going to happen. They can be taught by getting in the pool with them, putting your hand under there belly and helping hold the back end up while they paddle; they will get the idea. It takes all dogs different amounts of time to learn, I guess it depends on much time you spend in the water with them. I take my chi boating with me & swimming in our pool in the back yard, so when spring/summer comes--we practically live in the water. If you don't think you're going to have time to teach them to swim, they have doggie life jackets they can wear while they're outside by the river, but it really is important for dogs to know how to swim.