Hi Mija,
Only an opinion: There are several research papers from several contexts that document bacterial contamination of bottled water. These include many well-known brands.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5025974/
https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2672.1997.00353.x
https://www.researchgate.net/publication/290455821_Microbial_assessment_of_bottled_drinking_water_of_Kathmandu_valley
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6222228/
An emerging problem is also that of micro plastic contamination that seems to be prevalent in a majority of bottled water types.
https://www.theguardian.com/environment/2018/mar/15/microplastics-found-in-more-than-90-of-bottled-water-study-says
Boiling/ filtration is difficult, but where possible (IMO) it should probably be recommended equivalently for bottled and other water sources. If the situation makes boiling and filtration impossible and water must be used 'as-is', then I would rather go with bottled than natural water sources. If the climate is hot and sunny, natural UV decontamination may be an option.
That said, water would normally be heated for making formula in any case. I've never made formula without hot water so not sure how soluble it is at low temperature.
Other (expensive) potential options for decontamination are:
UV light wands
http://www.berkeleywellness.com/self-care/over-counter-products/article/sanitizing-wands-do-they-really-kill-germs
the life straw / filter bottles.
https://www.lifestraw.com/products/lifestraw
Regards,
Paul