You won't give the guard water. Go to the mansion in celadon city and talk to the lady that is sitting in the room. She will give you a TEA which you will give to the guard.
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go to the house with the old lady and talk to her and you will get a cup of tea then talk to the guard
Buy fresh water and give it to the guard
If your playing the original red version you need to just hand the water to the guard, if your playing firered you need to get Tea from an old lady in the celadon mansion and give that to the guard.
its not water its tea go to celsdon city and to the left of the Pokemon center there should be a biulding a old lady willbe there talk to her and she will give u the tea then take in to the gard then they all will be open
Give the guard a drink of water. You can get one from the vending machine in the Pokemon Mart in Celadon City. The vending machine is on the top floor. Once you buy one from it, walk past him and he'll ask you if he can have your drink. Say yes and you will be able to pass.