Many dogs naturally love to swim, others will learn to swim with a little prodding, and other dogs will simply never learn to swim- but that doesn't mean they can't be included in family swimming fun!
Here are some tips to help teach your less-than-enthusiastic dog to swim
For energetic, enthusiastic dogs, all it might take is jumping in the water and calling your dog to you. The dog will teach itself how to swim. Some dogs, however, will be thrown off by the splashing water- especially if it is chlorinated.
If reluctant, use a leash to help pull your dog into water deep enough that the dog has to swim rather than walk. (If you dog plants her feet, or refuses, it's best to take it slow, read the articles listed below, and take baby steps towards acclimating her to water) Most dogs will sputter and struggle a bit as they are beginning to swim, but will catch on within a few minutes. Stay near your dog and be ready to pull him back to shallower water if he continues to struggle.
A bit of helpful trivia here: cooked, hot, hotdogs float. Take advantage of your next cookout to give your dog a great good excuse to swim. Give your dog a little bite, and then toss into the water to entice him out.
Some dogs, even with these methods, still won't ever learn to, or ever feel really safe swimming. Consider a dog lifejacket. Most of the jackets are constructed like a sling with flotation devices on either side and straps to hold it in place. This setup is very comfortable for the dog and most dogs enjoy swimming with a lifejacket even more than swimming without. (bassets, daschunds, and other short-legged/long backed breeds should always wear a life jacket in deep water)
Remember that boating dogs should almost always have a lifejacket depending on the size of the body of water and the size of your dog.