If you don't want to join your dog in the pool or you aren't certain that he's a strong swimmer, fit your pet with a canine life vest. There are lots of options available (like the Kyjen Outward Hound Life Jacket), but look for features like grab handles to make it easier to pull your pet out of the water in case of an emergency.
The Benefits of Swimming for Dogs
Just like with humans, swimming is a great form of cardiovascular exercise for dogs. It's also easy on the joints, which is especially good for older dogs. If your pet suffers from a chronic condition like arthritis, hip or elbow dysplasia or obesity, he might benefit from formal canine hydrotherapy treatments that use heated water, special harnesses to keep the dog in the correct position and jets to add resistance. Ask your vet whether this type of exercise is right for your dog and for a recommendation to a qualified practitioner.
Just as you would slowly increase your own cardiovascular endurance (rather than plunging into the pool to swim for an hour on Day One), take the same precaution with your pet. Even if your dog is used to walking or jogging with you, swimming might be new to your pet, and he'll need time to adapt to the movements and build up endurance. Start with shorter water sessions before you let your dog swim for longer periods of time.
Health Risks of Swimming in Lakes and Streams
If you live near a like or stream where your dog plays frequently, there are several health risks. There are invisible bacterial and parasitic infections that both dogs and humans can contract from untreated fresh water: Leptospirosis and giardia among them. Your dog may contract these infections by drinking contaminated water, or even opening his or her mouth in water in which these organisms live. In addition, Leptospirosis can infect through any open cuts or wounds (which might not always be visible). Ask your vet about a vaccine for Leptospirosis. (The Giardia vaccine is commonly given only as part of treatment after infection.)
An even more dangerous and potentially deadly bacteria (that you can actually see) is blue-green algae, which produce toxins that can sicken and even kill humans, pets and livestock. There's no antidote for these toxins so prevention and vet care are crucial. Never let your dog swim in water that shows signs of algae bloom, which looks like mats of green material floating on the surface of the water.