go to cherrygrove. surf in the sea man gives 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.
The HM Dive is not in SoulSilver or HeartGold, for there is no deep water to dive into.
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.