Dasani is not spring water. It is just filtered water, most likely from a municipal source.

I don't like the added minerals, but that is a personal preference.

http://www.dasani.com/faq.html#what-is-dasani

Sometimes we are able to get the FA to give us a bottle of the spring water they often have in the drinks cart, but YMMV and depends on the airline and who catered the flight. The Dasani in F/J is I'm sure a contract item, it's always there at the seat on DL (and AA, if I recall correctly)