Most of the time, dogs are able to swim naturally, although for obvious reasons, it's not a good plan to have tiny puppies in water.
Like with humans, swimming is excellent exercise for dogs, especially if they have joint problems like arthritis, since it exercises most of the major muscles in the body, plus it's low risk since it's physically impossible for the dog to strain any part of their body when it's in water. Know however, some may need help due to being heavier, like English Bulldogs for instance.
Also, some breeds are particularly good at swimming, since they were originally bred for rescuing people in danger, like Newfoundlands and St. Bernards for instance.
Hope this helps.
they usually know how to keep their head above water, but not all dogs know how to propel themselves to the edge to get out. You have to teach them
They know how to make the paddling motions and will do that instinctively if they find their feet no longer on the bottom, but it takes them a little practice to figure out how to do it effectively
Dogs know how, not all dog like to swim and some dogs sink like a rock and can't swim no matter how hard they try.
Most can naturally swim, others need a little help. Some individuals of some heavy bodied breeds (like Bulldogs) will actually sink.