I recently cleaned out the waterers and added ACV. That was a day or two ago and the water has remained clear and fresh! What exactly is it doing for the chickens.. I just like that the water is staying fresh!
Depending on what kind you are using it has multiple benefits. If just plain ACV, it contains vitamins and minerals that can help with electrolyte replacement. It is also providing an acid in your water that discourages the growth of bacteria and fungi. It is providing an acid environment for the GI tract that can discourage parasite growth , but will not act as a dewormer.
If you are using unpasteurized and unfiltered ACV, with the full mother intact, it goes even deeper than that. Here's a link to some interesting reading on what this can do for your flock's health as a preventative method:
For me, when I started using it on a regular basis and not just casually to keep the water good or for a vitamin booster, my egg customers started noticing a change in the flavor and smell of my eggs...and I did also. They stopped smelling and tasting of sulfur and became buttery, nutty and sweet...very mild in flavor and no smell when cooking.
One of my customers(an old Mennonite lady) said she hadn't been able to eat eggs since she was a little girl (now 74 yrs old) without getting an upset stomach..but she could eat my eggs without any problems now.
Note...had one fella on city water report that adding ACV to his flock's water caused it to be too acid, as his water was already acidic, and said it caused his whole flock to develop gleet~thrush of the digestive tract..an abnormal overgrowth of harmful yeasts~but he was the only person I have ever heard of that happening with...and many, many use it on town water without any such outcome. YMMV