You can have Pokémon that aren't water types battle on water. For example, Pikachu. Huh, wonder how that works.
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Pokemon Logic
RandomPokémon logic at its finest. Warning: Do not drink any liquids while reading this, as you may laugh so hard that you choke XD Warning: do not read this at night, as you may laugh so loud that you wake up your family XD
Water battles
You can have Pokémon that aren't water types battle on water. For example, Pikachu. Huh, wonder how that works.