Yep, I second that! Ask for a "cup of ice water" or just an "ice water" and everyone at the CS restaurants knows what you mean. I go in every hour when it's hot, even if that's all I want to order. I have never had a cast member blink an eye, even if I'm standing in line at places like Pecos Bill's or Cosmic Ray's during a busy time. In fact, everyone seems happy to oblige.
Staying hydrated is the key to WDW in the summer time.