Not all dogs are swimmers - make sure you supervise your dog if near a lake. Also remember that your dog may drink dirty water and become ill from parasites or other undesirable contaminants. Some lakes and most pools are treated with chemicals - this is not a good place to get a quick drink. Dogs can get sunburn just like humans. Use sunscreen, There are products available for dogs, but a good hypoallergenic product can be used on noses and other exposed areas.
Summer is the season for insects.
Protect against flees, ticks and heart worm. Consult your veterinarian for recommendations in your area. Warm weather also brings on allergies from pollen, grass weeds and fleas, etc. If your dog is chewing on himself, consult with a veterinarian.