Why focus on water bottles? What about the tens of thousands of plastic packaged 'snacks' - 3 biscuits and a bit of dip that are served in multiple layers of plastic packaging? Or the tens of thousands of serves of cereal in plastic packets accompanied by plastic milk bottles? Or juices. Or cans of soft drink. Or plastic drinking cups. All of those could be replaced by more environmentally friendly packaging or serving containers. But it doesn't serve the airline to change those from a financial position.
let's pick on water, which we know can get contaminated either before delivery or while sitting in tanks in planes.