7. Q: Why won’t the Saffron City guard take my Fresh Water?
A: To prevent people from getting into Saffron early by trading in a Fresh Water, they’ve changed the item you need to give to the guard. On the first floor of the Celadon Mansion is an old woman who will give you the “Tea” item. This is what the guard will accept to let you pass.
You have to give him tea because since you can attach items in the newer games, you could just trade over a Pokemon with the beverage when you get to Cerulian City and get to a part of the game when it's too strong for you.