Unless your dog just immediately runs into the water and starts swimming, the dog may need some encouragement.
Buster Boy loves the water, but he didn’t take to swimming right away. What he did do right from the start was play around the shore and DIG! He can spend hours digging holes in the water bottom
I first brought him to a slow moving sleugh that is off the Willamette river and adjacent to a dog park. He loves to chase rocks or sticks, so I coaxed him into the water by throwing a rock a few feet from shore. If I threw it too far he would stop at the waters edge, but close to shore he would run into the water to where it landed. After a few times I would throw it a little further, and a little further yet until his feet did not reach the bottom and he would have to swim to get to the spot. It took about a month or two of doing this before he would just go into the water and swim on his own.
You need to make your dog feel safe and comfortable in the water. What you do not want to do is to just throw your dog in the water. That may seem obvious to some people, but I have seen people do this over and over again, and it is horrifying to me. The dog is terrified by the experience and most likely will never want to go in the water again.
Some breeds take to the water easily as it is a trait of their breed. Bull breeds with the pushed in nose have problems inhaling water, and some breeds are just not built for the water. Bull dogs cannot keep a float, and very short legged dogs often have a problem swimming.
Safety in the water
You should keep your dogs safety in mind at all times. Large breed dogs have more strength and endurance and can safely go further from shore than small dogs, but can still be swept away by currents. I have seen stories in the news about dogs being carried down a river and the dogs owner drowning from attempting to rescue the dog. The ocean can have strong undercurrents. Near still water is the safest. One way to keep your tog in tow is to still have them on a long retractable leash while in the water.
I also advise people to carefully look at the surroundings before allowing the dog in the water. Avoid it if there is a good amount of litter and garbage around the shore. Avoid water that is heavily wooded with fallen branches that could tangle the dog. Is it a common fishing area? You want to look for fishing lines that may have hooks on them. If the pond is heavily infested with algae it may contain blue-green algae which is actually a poisonous bacteria if ingested. Depending on where you live in the country there can be dangerous animals in the water: snakes, gators, and even large fish such as gar have been reported to bite small dogs.
Swimming pools are safe as long as the dog can easily climb out of the water. You need to train the dog to know where the exit point is and through repetition that the dog can do it confidently.
Many dogs will go into the water to fetch, but Buster is more unique because he likes to swim just to swim. He will go out and just swim around in circles. He has created his own game in the water: he will bite at the water, or thrash his front legs to make bubbles and then snap at the bubbles! He has done this for two hours before coming out of the water.