Instructions
Travel to Undella Town or Undella Bay. The quickest way -- immediate transport -- is to use a Pokemon that knows "Fly."
Walk around Undella Town or Undella Bay until you see a "shaking spot" in the water. These are uncommon, but you should not have to travel long before seeing one.
Travel into the water using the Pokemon that knows "Surf." The "shaking spot" will disappear if you get attacked by another Pokemon, so consider using a "Repel" to protect yourself from unwanted encounters. With "Repel," you will still be able to encounter Wailmer.
Touch the "shaking spot" to begin the battle. Use electric- or grass-type Pokemon to weaken Wailmer, but do not kill it. Once Wailmer's health is in the red, throw a Pokeball to capture it.