Fairly self-explanatory, I suppose.
Which is more effective at rehydrating a human: chilled water (less than room temp) or room temperature water?
The only difference is how much it will decrease your body temperature. When dealing with a glass of water this ranges from insignificantly to very insignificantly. In both cases it goes into your stomach, heats up to body temperature, and later gets absorbed.
“He has ice water in his veins” is not meant to be taken literally. 