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Hydro Pump is the most powerful attack with the least drawbacks for many Water-types. To say that Hydro Pump is one of the best attacks in the game wouldn't be an overstatement. 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 Surf instead of Hydro Pump to ensure that their attack will always hit.
Surf is a weaker but more accurate option.