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Deals damage, but does less damage as the user's HP decreases. Base Power is calculated thus:
Base Power = 150 * (User's Current HP / User's Maximum HP), rounded down, but not less than 1.
This move has the potential to be immensely powerful as long as you can keep the user from being damaged. It is often used on Kyogre to deal massive damage to the opposition – between Choice Specs, Drizzle, a base 150 Special Attack stat and a 150 Base Power STAB move, there is basically nothing in the game that can stop it; however, in the long run, Hydro Pump and Surf are both more reliable options. For example, below 80% health, Hydro Pump is stronger, and below about 63% health, Surf is stronger. Most of the other users, such as Blastoise and Jellicent, are defensively inclined, so prefer Surf.
Surf is a more reliable move, but its base power is lower than Water Spout's base power when the user's HP is full.
Hydro Pump is a more reliable move with imperfect accuracy, but its base power is lower than Water Spout's base power when the user's HP is full.