If you're in Australia contact the Animal therapy special interst group of the APA. Also, Griffith uni in Q'land have a Masters in Animal therapy programme - they can put you onto lots of useful info. The UK physio society also have an animal therapy group.
Outcome measures will depend on what the problem is - function is always the first measure. be aware that mammals breathe differently when swimming to humans - particularly if not accustomed to swimming. So keep them to laps with a rest at the end of each, don't leave them in the pool swimming continuosly or they can quickly become distressed. Once they are comfortable in the water you can extend swim times. We used to swim horses, into the pool, through, out and trot round to the start again, we had a straight pool. Use a harness with 2 leads attached at the area between shoulder blades. A person on each side to guide them stratight in the beginning. When they have the hang of it you can reduce to one person/lead.
Benefits include: same as for reduced weight bearing exercise.
Importantly, in a dog with a limited range of motion they will increase range because they have to stretch further to stay aflot and upright - so you don't want them swimming if significant inflammation/trauma as will just increase tears etc. It also tends to make them even if say have a shortened stride on one side - treadmill work has the same effect. You can use a treadmill in water for walking/jogging, which reduces range needed. If the joint is just stiff, swimming is brilliant. Don't overdo it!!

Heidi Roland-Kenn (Oxford, UK)