Manaphy teaches Ditto the ability Dive to enter Bubbly Basin in Pokémon Pokopia.
Bubbly Basin is the first area introduced through the Pokémon Pokopia Expansion Pass. It is an underwater region located at the bottom of the sea, so Ditto needs a way to reach submerged areas before the new story can begin.
The route opens after Ditto encounters Manaphy, which teaches the Dive ability. At Bleak Beach, Popplio then leads Ditto through a submerged undersea gate into Bubbly Basin. Once there, Ditto is asked to help construct a large ocean temple while improving the underwater town.
Dive is easy to confuse with Surf because both are established Water-related abilities in Pokémon games. Surf is commonly associated with traveling across water, while Dive is the specific ability used to access submerged locations in Pokopia. Bubbly Basin is based on a sunken Cerulean City, which was submerged by rising sea levels.