I have recently moved to a new vet and she did suggest it. My lab is 5 years old and is a show type, whereas my other lab is a working dog and is always lean no matter what he eats.
I have been cutting down on his previous food and it has made very little difference, I also believe that by giving a dog less that required food means that the body stores it for longer and that they will be getting less essential vitamins than necessary.
I think the main issue is exercise as he will go for a long walk, but just plods and sniffs, whereas my working lab is constantly running about. Swimming sounds great, but not easy as I do not live near the coast or have a swimming pool, but I will search for hydrotherapy.