I suppose it is possible that you are getting something from the tap water. Have you tried chilling it in the fridge / letting it settle overnight? Maybe adding a drop or two of lemon juice. You could get a Brita water filter (I have one because I don't like / trust the water in my building).

Dry mouth can be because of a bunch of things... But drinking coke with the sugar and salt in it isn't so good for that (but the sugar is surely addictive)... Sounds to me like... YOu are starting to get addicted to coke ahah. Seriously... I used to live off coke without drinking any water at all... Water was actually repulsive to me... Until I moved to Australia and learned that I had to to be hydrated. Not drinking water... Will make your mouth drier. Dry mouth / throat will cause irritation...

A sugar free throat losenge may help stimulate the flow of saliva.