Carrying water during touring?
I have used old wine cask bags (or bladders, readily available in Australia, not too sure about rest of the world) packed either into a pannier or strapped onto the top of the rear rack (not the best position but I was fully loaded).  Some would say the slight after taste of wine is a bonus.  I have also seen unused bladders available from some camping stores.

I also have carried water purification tablets just to be safe.