On the third level "Toba Coastline" you will need to select Hayato, Aiko & Yuki. Have them jump into the water and follow the narrow passage until it opens into the wider water section. Follow the coast to the left-hand side to find a small island with one large rock pillar in the middle. Follow the small path around the base of the rocks to find two umbrellas. A small conversation will start and the achievement will pop.