This is a side quest based achievement. Simply Complete the "Dolphetta Stone" quest. This is available after you turn in the first 3 shards and can access the 4th row of tiles. You start the quest in Stone Fish Village. Speak to the white witch with red glasses just south of the portal pad.
https://imgur.com/FUDfxwI Witch Example
You've gotta find 5 dolphins and play a disrespectful audio clip for them. They appear on the map as a ! when you uncover the area they are contained in.
https://imgur.com/NuMN3se Dolphin Locations
The circles represent dolphin locations (just equip a swimming or floating form and interact with
)
The triangle represents the quest giver's location. After returning to the white witch and learning the horrible secret of what the dolphin has been saying to you, the achievement unlocks.
Happy Gunting