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Deals damage and has a 30% chance to lower the target's accuracy by one stage.
Muddy Water is generally inferior to Surf, which is more accurate and has the same Base Power. Muddy Water's secondary effect is negligible too: lowering accuracy is an unreliable tactic since the opponent can simply switch out and reset any changes made.
Surf is a more reliable option.
Hydro Pump is a more powerful version of this move with 80% accuracy and no side effect.
Scald is a less powerful move that has a chance to burn the opponent.