It is in Cherrygrove City. You will need a pokémon that knows Surf and the appropriate badge. To the south west of the city is some water, surf there and talk to the guy standing on the island, he will give it to you.
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The Castform that you get from the guy at the Weather Institute has a Mystic Water attached to it. When held by a Pokemon,the Mystic Water raises the power of its Water-Type moves.
When held by a Pokemon its raises the power of water attacks.
Have surf and go to Cherry Grove city and swim across the lake and talk to the guy. He will give you the mystic water item
Mystic Water increases the power of all Water-type attacks however in Pokémon FireRed, the increase in power is only a 10% increase.
go to cherrygrove. surf in the sea man gives it to you.