first you must have a Pokemon that knows surf ( if you don't know what this is Google it), then go or fly to cherrygrove city and use surf to the water( which you can find to the west). then if you go a little bit to the west you will find a guy on a little island. Talk to him, he will tell you something and then he will give you one, if im not mistaken. if you want advice of which Pokemon to give it too, give it to Suicune or Lugia, but that's just my advice.
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go to cherrygrove. surf in the sea man gives it to you.
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