My dogs are in the lake daily and who wouldn't be with this summer heat. I have to get into the vet but they have all started scratching and losing patches of hair. Mostly on their underbody and their legs from the knee down. Surely I'll get them looked at but has anybody seen this? I am sure it's lake/heat related. At least I hope it is.

Your dogs could have picked up ring worm, mites, lice or numerous other creatures that live in the tall grasses and murky areas around ponds. Ponds themselves carry all sorts of problems that our dogs are exposed to but who can stay away from them. Labs love to swim and we can't deny them that pleasure. Hope you find an easy answer.

I thinkn you are describing Pond Itch. The water sits on the coat and with the additional heat, the bacteria in the water create a staph infection on the dog's skin. My vet said you can't swim dogs in local ponds, lakes and streams until the water gets cold enough to kill the bacteria in it. We live in dirty times:)
With that said, I have found success with hosing the dogs down after a swim AND pouring a very dilute solution of household bleach and water (1 part bleach/32 parts water. . . you should not even smell the bleach) over their backs. Mine get this Pond Itch mostly down their backs.
Good Luck!

We call that "duckage" here in NYS. It is a bacterial infection most likely. With little rain to keep the water levels high in ponds and lakes the bacteria and fungal infections will rise. They may need to be on Cephalexin and antibacterial baths(Liquid Dial Soap, or a vet's prescrip. shampoo)for a couple of weeks in addition to keeping them out of the water.