What is Hydrotherapy?
The dictionary describes Hydrotherapy as: "The internal and external use of water in the treatment of disease".  In terms of use for the healing of canines it is: "The controlled use of buoyancy and resistance of water for therapeutic reasons."
Swim therapy is often used as a form of gentle non-weight-bearing exercise for conditioning, weight control and rebuilding of muscle. The use of water in a controlled environment can also be used to build cardiovascular fitness in the canine athlete.  It can be a great help in the rehabilitation of an injury post- or pre-operatively.  Hydrotherapy can be combined with other therapies in the management of chronic disease as well as chronic injuries.  Swim therapy is based on buoyancy and resistance. The warm water and weightlessness of the dog relieves the pressure on the joints and allows the muscles to relax.  Resistance of the water increases range of motion with minimum pain.  Results are safe, quick, and usually sustained.  Swim therapy is also a terrific way to keep your dog in peak physical and mental condition. It is not the be all and end all and should not be treated as such.  This is a very effective form of therapy used in conjunction with other modalities of physiotherapy and veterinary care. 
Hydrotherapy offers a therapeutic, relaxed and enjoyable workout. Many dogs are not adequately exercised in day to day life, especially if suffering from arthritic joints or after an injury or surgery. Controlled swimming allows muscles to be stimulated and exercised without the stress element associated with land-based exercise, which potentially can cause pain.  Vets often recommend a course of hydrotherapy for pre- and post-surgical conditioning, weight reduction in obese animals, painless exercise for pets with arthritis or dysplasia, cardiovascular workout for seniors, rehabilitation for stroke sufferers and pain management.  It is therefore of use in many orthopaedic conditions both pre-operatively to improve muscle tone of affected limbs (for instance prior to total hip replacement) and post-operatively to facilitate healing (for instance in post-operative care of cruciate ligament rupture or fracture repair where light swimming can begin as soon as the sutures have been removed).  Hydrotherapy, if initiated early in the course of disease, can help to maintain muscle tone in cases of degenerative and neurological diseases.  Hydrotherapy provides an effective means of maintaining fitness and toning muscle, particularly in the winter months, for dogs training in competitive athletics.  It is also a valuable aid to the professional handler. 
Immediate Benefits
Hydrotherapy techniques help to relieve pain while strengthening and re-training muscles. The animal is effectively weightless when swimming. Consequently a full range of joint motion is encouraged, thus improving muscle tone without imposing undue stress on damaged tissues. This is how results can occur so quickly:-

Conditions Conducive to Hydrotherapy
Canine Weight Loss Program
Water Training for Working Dogs
Hip Dysplasia
Arthritis
Neurological Conditions
Depression
Maintenance Programs
Endurance Programs
Muscle Development and Retraining
Pre- and Post-Surgery
Torn Ligaments and Tendons
Soft Tissue damage
Geriatric conditions (atrophy, arthritis)
Improve joint function and increase the range of movement
Spinal injuries
Relaxation of muscle spasms
Paralysis
Improve circulation