Strong Counter Types
* Electric: Electric-type attacks are super effective against Water-type Pokemon. This is your best bet. Examples in Diamond:
* Pikachu: Learn Thunderbolt, Thunder, and other powerful Electric moves.
* Luxray: Powerful Electric attacker.
* Electivire: Another strong Electric attacker.
* Raichu: An evolved version of Pikachu, also with strong Electric moves.
* Grass: Grass-type Pokemon are also super effective against Water-types.
* Grovyle: Learn Razor Leaf, Solar Beam, and other Grass moves.
* Sceptile: A powerful Grass-type.
* Torterra: Has Grass/Ground typing, giving it a strong defensive advantage.
* Roserade: Can learn strong Grass moves, but be careful of its low defenses.
* Dragon: Dragon-type Pokemon are super effective against Water-types.
* Gible: Learn Dragon Claw and Outrage (though be careful of Outrage's side effect).
* Salamence: A powerful Dragon-type with high Attack and Speed.
* Flygon: A strong Dragon/Ground type.
Other Effective Options
* Fighting: While not super effective, Fighting moves can still damage Water-types.
* Lucario: A powerful Fighting/Steel type, can learn strong Fighting moves.
* Machamp: Has high Attack power.
* Breloom: A fast and powerful Fighting/Grass type.
Tips
* Type Matchups: Pay attention to type matchups. Don't assume a Pokemon is weak just because it's Water-type. Some Water-types have other types for coverage.
* Move Choice: Choose the right moves for the situation. Electric moves are your best bet against Water-types, but remember to use moves that match the opponent's weaknesses.
* Status Conditions: Status moves like Thunder Wave (paralysis) or Toxic (poison) can hinder Water-types and give you an advantage.
Remember: The best strategy depends on the specific Water-type Pokemon you are facing. Be sure to research the individual Pokemon's weaknesses and strengths!