Coke and Pepsi often deliberately compete with each other, e.g., Coke bought Minute Maid so then Pepsi bought Tropicana. I don't know whether Dasani or Aquafina came first, but some smart guy figured out that it would be easy for them to get into the water business.
They were already purifying water so that the cola would taste the same everywhere. The other end of the plant had bottling machines, so all they needed was a hose to go around the soft drink equipment. New labels, the distribution was already in place, really a turnkey business.
The odd part is that a pint of water sells for more than a pint of cola!