Game Freak Logic.
Pokémon is first and foremost a kid's franchise, with competitive players (and logic) taking the backseat. So logically, yes, Charmander would die when hit by Soak... or Surf... or Brine... you get my drift. However, since Pokémon don't regualrely die in the anime (although it is referenced that Pokémon will fight to the death in the first Pokémon movie, and Latios dies in Pokémon Heroes for a worthy cause) and you can't actually witness the death of a Pokémon in the games (although it is also implied that they die), they can't go around killing Pokémon willy-nilly, or else sales will drop and Pokémon will go under and we will all be very sad.
Post-script: The Manga is the exception to the rule, where both people and Pokémon can die or be killed. For example, in one manga it is revealed that Team Aqua Leader Archie killed Team Mamga Leader Maxie in a sword duel. Not someting you'd see in the anime or the games.