You get this on Disc 3.
Get an Ocean Chocobo through playing Chocobo Hot'n'Cold.
Go to Quan's Dwelling outside Treno.
Head though the cave and outside with the clock.
You can select to use a dead pepper on the edge of the walkway.
Press Y when you're on the world map to select the dead pepper.
(You can also just walk over to these bubbles on the world map)
You will dive into the bubbles below and you will unlock the achievement.