If you're staying on the Strip, become friends with the nearest Walgreens. There are several scattered up and down the Strip including this one across from Treasure Island. If nothing else, sticker shock from hotel gift shops will drive you there. These Walgreens cater to tourists and the front of the store looks like a supersized convenience store with a huge array of beverages, fountain drinks, sandwiches, fresh fruit and snacks at prices close to what you'd find at home, not the rip off prices in the hotel gift shop. Younger crowds were toting cases of beer back to their hotel rooms.
There are lots of ways to spend money in Vegas, but $3 for bottled water shouldn't be one of them. Head to Walgreens instead.