The Pokémon types that beat Water are Electric and Grass. Water-type Pokémon are weak to those two types, so they deal super effective damage.

Quick answer

  • Electric : A very common counter to Water.
  • Grass : Also super effective against Water.
  • Special note : Some Water Pokémon with a dual type may change their matchups, but these are the standard weaknesses.

If you want, I can also give you a simple Pokémon type chart for Water matchups.