Pokémon Pokopia's Bubbly Basin DLC includes six treasure maps that point to decorative Poké Ball items, which are required to build a mermaid's gym habitat for Starmie.

The maps are obtained by digging glowing spots on the Bubbly Basin map and are available only to players with the DLC. The treasure locations can be reached without using the maps.

Treasure Map #1 rewards a Decorative Great Ball and is found beside the Pokémon Center in Bubbly Basin, after descending stairs on the left side of the bathing house and destroying a wall of blue rocks behind a bench. Treasure Map #2 rewards a Decorative Heal Ball and is located northeast of the ocean temple, after swimming past a tall red‑roofed building to the area where Barboach appears during the request "Build the Beautiful Building!" and breaking ground blocks near a large coral. Treasure Map #3 rewards a Decorative Dive Ball and is found east of the main square, after breaking a wall of dirt blocks to reach the abandoned gym, turning south and destroying a wall of star‑spotted rocks to enter the room with the Dive Ball. Treasure Map #4 rewards a Decorative Net Ball and is located in the cave northwest of the main square, the same area where Frillish appears during the request "Build the Beautiful Building!"; after entering the cave and jumping back into the water, breaking net blocks and proceeding past tracks and blue crystals, the wall of blue rock on the southeastern side can be destroyed to access the room with the Net Ball. Treasure Map #5 rewards a Decorative Lure Ball and is found near a house with a yellow mailbox on the northeast side of the map, close to portal pods, by digging on the west side of the building near coral. Treasure Map #6 rewards a Decorative Quick Ball and is located in the southern part of the map near the Bubbly Basin gate; traveling north, descending stairs, turning west and breaking star‑spotted rocks behind a ruined house leads to the room containing the Quick Ball.

To create the mermaid's gym habitat, place the six decorative Poké Balls on the six exhibition stands found inside the abandoned gym east of the main square, then locate a marine sofa, which can be crafted using a known recipe.

Additional guides for portal pods and a Starmie doll are available, and a Bubbly Basin habitat dex has been updated.