Instructions
Obtain one of the many water type Pokemon. A few Pokemon able to learn Hydro Pump are: Squirtle, Wartortle, Blastoise, Psyduck, Golduck, Poliwag, Poliwhirl, Tentacool, Tentacruel, Horsea, Seadra, Staryu, Gyarados, Lapras, Vaporeon, Omanyte, Omastar, Totodile, Croconaw, Feraligatr, Chinchou, Lanturn, Marill, Azumarill, Qwilfish, Kingdra, Suicune, Lugia, Mudkip, Lombre, Pelipper, Wailmer, Wailord, Milotic, Huntail, Gorebyss, Relicanth, or Kyogre. Most of these Pokemon can be captured in the wild.
Train your Pokemon by battling wild Pokemon, other trainers, Gym Leaders and the Elite Four. Some items can double experience points earned in battle, split the experience points between Pokemon, or null points received. Use these items to your advantage.
Level up your Pokemon. Refuse leveling for certain Pokemon, like the starter Mudkip, by pressing the "B" button when the evolution screen appears. Some Pokemon must evolve to learn Hydro Pump, so consult a guide for the specific statistics for your Pokemon. Once you meet all requirements, a message will popup stating your Pokemon wants to learn Hydro Pump, but already knows four moves. Delete a move and learn Hydro Pump instead.