The biggest selling brands, Zephyrhills, Dasani, Evian are indeed tap water, They just take out the chlorine. It is no better than tap water, and has to undergo fewer inspections.
When I was in college, I worked in a Pharmacy and we bought 5 gallon bottles of distilled water, and my job was to pout it into gallon jugs, and we sold them for 50¢, of which 15¢ was a deposit on the gallon jugs. In a town of 10,000, we sold about ten bottles per week. The main customers were women who used it for their steam irons, and two local hypochondriacs. The local grocery did not even dell distilled water or water in jugs, unless it was fizzy.
The marketing of water is one of the greatest ad campaigns (ie scams) in the history of Humanity.