A dog is more bouyant in water. The water holds you up. In fact, if you stop stroking and kicking you won't sink more than a couple of inches below the water's surface, if that much. For those who shouldn't be working their joints too hard, swimming is considered the best exercise. The photo above shows that an English bulldog can swim. Some dogs take a while to figure it out, but they can swim if they just stop worrying about it. I've had two dogs who were afraid of water, but learned to swim.