From Bulbapedia, the community-driven Pokémon encyclopedia.
The Water Deck is a theme Half Deck from Pokémon Play It!. It predominantly focuses on -type Pokémon.
In Pokémon Play It!, it is one of four decks available for the player. Julie always uses the Fighting Deck against it. She uses the deck herself should the player choose the Fighting Deck.
In Pokémon Play It! Version 2, this deck appears on the first and second "Basic Challenge" levels, which are a Fighting vs. Water and Water vs. Fighting duels, respectively.