When Fluttershy is diving under a lake, a mysterious voice speaks to her and gives her seven mystical keychains that allow her and her friends to breathe underwater, mentioning that the treasure of the Pirate Trio is in this lake. Of course Rarity jumps at the idea to look for it, leading them to an underwater adventure.

After watching the trailer of them at the beach, combined with rewatching the movie with the whole sea pony scene, I was suddenly inspired to do this (despite how reluctant I was to watch the episode due to the hell Sunset will go through), as well as voicing a headcanon I have that not many people would think of (though like my Storm King story, I don't put this story in my headcanon).