Jumping into the water is a great way for your dog to cool off, especially on hot, steamy days. Swimming can also be a great everyday activity, since it's easy on the joints, but never force it on your pup if he isn't a fan of the water.
If he is a fan, always be cautious of water quality, and don't let him drink from the pool, lake, stream, or ocean. Swallowing too much salt water or chlorine can very quickly make your dog sick.
Keep a close eye on him while he's paddling around, and help him steer clear of deep waters and strong currents.