Manaphy teaches Ditto the Dive move in Pokémon Pokopia’s Bubbly Basin expansion.
Dive is the key ability needed to reach the underwater area added in the first Expansion Pass part. After Manaphy teaches Ditto the move, the player can access submerged locations that are otherwise unreachable from the surface.
Popplio also has an important role, but it is not the Pokémon that teaches Dive. Popplio appears at Bleak Beach and leads Ditto through an underwater gate into Bubbly Basin. This makes Manaphy and Popplio easy to confuse when recalling how the expansion introduces its underwater setting.
Bubbly Basin is a sunken version of Cerulean City, located at the bottom of the ocean after rising sea levels transformed the landscape. Once there, Ditto’s major project is to construct a large ocean temple at Popplio’s request.