Class action suit against water companies?:
There are literally tens of thousands of water companies in the US.

They are generally regulated by various federal and state agencies. If they comply with those regulations, there is no basis for a suit, class action or otherwise.

In order to prevail, you would have to prove negligence. 

If you don't like tap water, drill your own well, install your own filtration system or buy bottled water. But, you will not prevail if you attempt to sue a water company unless you prove they are negligent.