Open the Power Bomb door in the Kihunter Room, charge a shinespark, and go through the door to the Moat. Spinjump towards the pillar, fire a shot at the door, then activate the shinespark with Up or Angle Up, let go, press Right, then press Jump. If you time it right, the shot will hit the door and open it before you fly through.

The shot is most commonly a Super Missile, but can be a Missile or beam shot, but these have different timings.

The much easier alternative is the two-spark, starting the first spark in the Kihunter Room, then doing a separate spark in West Ocean. This is slightly slower, but much easier, and saves the health of sparking across the first part of West Ocean.