Drowning:

Even the best swimmers are at risk for drowning in a swimming pool or lake.  Prevent free access to water sources, and always supervise your pet when he or she is swimming.

Infections:

Leptospirosis (a bacteria) and giardia (a parasite) are prevalent in rural, suburban and urbanized areas. 

Leptospirosis is a bacteria that is passed by way of urine of livestock and wildlife (deer, raccoons, opossums, skunks, rats and other rodents).

Giardia are protozoan, tiny, one-celled parasitic life forms with the potential to cause serious illness. Some dogs are carriers who show no symptoms, but others get sick and need treatment.

Both can be present in any stagnant surface water, most soil, and recreational water sources such as ponds and lakes, although this is not the only way these are transmitted.

To help prevent both these, minimize contact with fresh water and mud that might be contaminated.  Always have water with you for your pet, and discourage drinking of stagnant water.  It is also recommended that your dog be vaccinated for leptospirosis.