Squirtle and and other water Pokémon are frequently depicted as making attacks that materialize larger volumes of water than could realistically be stored in their bodies. Is this water ever safe for human consumption, and if so, is it conditional on the type of Pokémon or the source of the water? (Do sea Pokémon produce salt water? Do poisonous water Pokémon, eg Tentacool, produce poisonous water?)
Some (possibly all) water-type Pokémon produce potable water
In the ninth Pokemon movie, Ash and his companions are thirsty. They come across Lizabeth who's using a water-type Pokémon (Poliwhirl) to produce water which her psychic-type Pokémon (Meditite and Medicham) are making levitate into spherical balls.
Max, Ash's companion, drinks directly from one of these spheres and suffers no ill health as a consequence. Additionally Lizabeth, who is familiar with water-type Pokémon and Ash, who's spent multiple years with a water-type trainer don't offer any discouragement or warnings to him before he drinks the water.
You can find your answer here in this post:
The conclussion seems to be that yes, you can drink water pokemon's water but better if you use some filter.