You know man, you're wrong.  I used to make water when people asked and never charged.  But as time went on, I just got tired of the ungrateful jerks couldn't even say thanks, let alone not tip.  It's common courtesy to tip, period.  If you expect the mage to stop what he'd doing, then you better damn well tip him/her, it's the right thing to do. 
  And your "buffing" comparison is just silly.  I buff people all the time when they run by and if they don't say thanks it doesn't bother me at all because I CHOSE to buff them.  Getting a whisper asking to make water is totally different.  And at the beginning of a BG if somone asks for water, they get none.  There is no excuse for them.  My shammy always visited the vendor to stock up on water.  I'm not wasting my time cuz some people are too lazy or cheap to do the same.  If its a long BG and they run out, I don't have a problem with that, I'll make em some.