Which Pokémon Pokopia expansion adds the underwater Bubbly Basin area?

Expansion Pass Part 1 adds the underwater Bubbly Basin area to Pokémon Pokopia.

In the expansion story, Manaphy teaches Ditto to use Dive. Popplio then appears at Bleak Beach and guides Ditto through a submerged undersea gate into Bubbly Basin. The new region lies at the bottom of the sea and requires Dive to access.

Bubbly Basin is based on a sunken version of Cerulean City. Rising sea levels have submerged the settlement, leaving trenches and darker depths where sunlight is scarce. Popplio asks Ditto to construct a large ocean temple, extending the game’s central building and habitat-restoration formula into an underwater setting.

Pokémon Pokopia has a paid three-part expansion pass. Bubbly Basin is the first part, while the other two parts were scheduled for later releases. It is therefore more precise to identify the content as Expansion Pass Part 1 rather than calling Bubbly Basin a free update or a base-game location.