You can find it in the fourth puzzle of the Ruins of Alphs.
The fourth puzzle, Omanyte, can be accessed after you have HM03 - Surf and HM04 - Strength. Like the third puzzle, you must go through Union Cave. Instead of exiting through the northermost opening on the second floor, head west after Surfing and use Strength on the rock to get past and into the final cave.
To enter the secret room, U must have a waterstone in your bag.
Then you'll see 4 pokeballs. One of them contains "Mystic Water"
I hope it's a good answer.
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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