Seabed

After falling down a pipe, you'll be at the bottom of the Seabed. To climb out, hold Up + B + A. Alternatively, press Down to drop into the ocean bottom.

Swim to the Save Album and use it. Read the sign to learn how to swim underwater and move quickly by swimming together.

Swim to the west area and check the right wall. Use Mario's Firebrand to light an empty flame, which shrinks the reef and allows you to proceed.

Go north and west into the next area. Check the map on the wall before continuing.

Follow the path to the edge and swim together across the gaps between platforms. Watch out for or fight Goomdivers that fall from above.

Swim to the southwest and enter that room. Use Luigi's Thunderhand to power the orb on the ledge, opening the large door.

Enter the next room. Swim across the reef pit, avoiding Bloopers. If you battle them, tap A + B after the fight to avoid falling.

Swim to the northwest and drop onto a piece of land below. Enter the northern room.

Go up the path and hit the wall switch, activating 5 flames. They will light in a specific order. You must have Mario light them with the Firebrand in the correct order: 1, 5, 3, 2, 4.

Once the flames are lit correctly, the yellow reef to the south will disappear. Swim into that room.

Swim around and fight some Bloopers. Then, go out to the southwest area.

Use the Save Album.

Enter the pipe and press Up + B + A to climb to the surface. A scene will play showing Bowser, Cackletta, and Fawful at the border, with Bowser collapsing and Fawful merging Cackletta into Bowser's body. The scene then returns to the brothers.