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Hydro Pump is the most widely-distributed, most powerful attack with the least drawbacks for many special attacking Water-types. When boosted by the rain provided by Drizzle Politoed in OU, one of the most defining characteristics of the metagame is to be sure that at least a couple of your Pokemon can take a STAB Hydro Pump from the likes of Choice Specs Politoed, Life Orb Starmie, and Choice Scarf Rotom-W. To say that Hydro Pump is one of the best attacks in the game wouldn't be an overstatement. Hydro Pump is also the sweeping attack of choice for many special Swift Swim sweepers in the lower tiers, including Kingdra, Ludicolo, Omastar, and Gorebyss. However, Hydro Pump does come with one major drawback: 80% accuracy. This means that it can, and will on occasion, miss at the most inopportune times. Therefore, many players will opt to use the perfectly accurate Scald or Surf instead of Hydro Pump to ensure that their attack will always hit.
Scald is a much weaker but more accurate option with a chance to burn the target.
Surf is a weaker but more accurate option.