How do I get to the Power Plant?
Accessing the Power Plant is optional at this point, so don't worry about that right now. :)
How do I get past the thirsty guards?
Go to the roof of the department store in Celadon City. There, you'll find a vending machine with Fresh Water, Soda Pop, and Lemonade. Buy any one you wish, it doesn't matter, and walk by the guard. He'll somehow know you have the drink, stop you, and take it. He'll drink it up, stop being thirsty, and let you pass as a way of thanking you. He'll share it with the other guards, too, so you only have to give a drink to one.
Hope I helped. :)
Source and Source