The physiotherapist is often in the tank with the dog to apply therapy & ensure correct movement.

When dogs are familiar & comfortable with the machine, they are able to swim/walk/wade by themselves, with the owner or assistant just holding the lead from outside the tank. Swimming is excellent exercise, however, the underwater treadmill offers a very different experience compared to a swim at the beach:
it is controlled
safe,
and we make sure the dog is moving correctly that is, 

Dogs are cunningly good at compensating, they can drag themselves through the water mainly using their forelimbs, and not use their back legs efficiently, they cheat if not watched.

Each dog needs a different programme according to his/her condition, so it is not just letting the dog go in the water! However, benefits derived from the treadmill:
strength,
endurance,
mobility, are transferred to other activities e.g. you will see an improvement in walking, swimming, running and co-ordination.