In Pokémon: Magikarp Jump, Mystic Water is a Friendship Item that is received as a reward after beating the Quick League. It summons Piplup as a helper Pokémon.
In SM040, Ia and Kanoa went on a treasure hunt for a Mystic Water in a sunken ship, with Ash, Lana, and Mallow tagging along. After Kanoa found it, he gave it to Ia as a necklace.