Underwater Plaza

Walkthrough: As you begin your way to and through the Underwater Plaza, there are a few Power Moons you can get right away: above the entrance and on the back of Dorrie. After you do, swim down from the entryway to the Plaza down below. You should be able to spot a location with multiple entrances, and when you do, take the top entrance to come out onto a balcony to revert back to Mario. The Lochlady nearby will advise that you can reach the surface by using the elevator-like tube in front of the two of you. So, run around the back and activate the elevator by using the P-Switch. This will cause the tube to fill with water, the same Mario can use to swim to the top.

Once up top, go to the nearby P-Switch to earn more Purple Coins; there will be even more Purple Coins on the water's surface where the platforms descend. From there, use the flower in sight to hop to another platform with wooden crates, the same that hold a Power Moon. Follow this platform to the other side of the surface area where it rests on top of a wall. Use Cappy like before to unzip the wall and find even more Purple Coins. Use the flowers on the balcony to reach the last couple of Purple Coins in this given area, and then leap across to the tower face the next of the Broodals.