In this entry, we finally save the Seaside Kingdom from a giant purple menace and restore the Sparkling Water.
In order to enter the lighthouse, you'll need to go underwater and possess a Cheep-Cheep. Once you do, enter the opening in the ground that leads into the underwater tunnel. You'll be able to reach the other side of the tunnel then.
You'll be back in Mario form once you go inside, but you'll be able to possess another Cheep-Cheep with Cappy as soon as you go in. Watch out for all the eels that pop out of the ground, walls and ceiling. Don't go too fast, but don't go too slowly, either. This will keep you from getting hit. There is not only a Power Moon hidden in a crevice in a wall (see Power Moon #39 below), but also two instances where you'll find Regional Coins (see 34-36 and 37-39 below). When you reach the stacks of crates, you've reached the end (as well as the second set of Regional Coins, which you'll need to turn back to find). You can then get the Power Moon on top of the lighthouse and ground pound the switch there to restore the fountain.
When you restore all four fountains, you'll knock the purple octopus off the Sparkle Water glass. This will start the boss battle with Brigadier Mollusque-Lanceur III, Dauphin of Bubblaine (Mollusque-Lanceur for short), who is really a giant Gushen. You'll need to catch up with him with a Gushen and hit him in his lava-soaked head with water from the Gushen's upwards flight. Afterwards, you'll need to keep hitting him in the head with water. At the start of the battle, he spits out spiky balls which can sometimes contain Hearts to refill your health with when they explode. He'll eventually shoot some cone-shaped missiles. Once you soak Mollusque-Lanceur's head with enough water with a Gushen, he will eventually explode, and Mario will be launched into the Sparkle Water glass with a Power Moon while the water from the fountain finally flows into the glass. The only way out is to grab the Power Moon. From there, you'll have free reign to do what you need to in the Seaside Kingdom.
Power Moons
2. The Lighthouse Seal - You'll need to go through the underwater tunnel to the lighthouse and go through the pipe at the end to get this Power Moon.
3. The Hot Spring Seal - There is an island with lava all over it called Hot Spring Island. Use a Gushen and use the upward blast to cool down all the lava. Not only will you find nine Regional Coins here (see 56-58, 59-61, 62-64 below), but you'll also find a Power Moon. You'll need to get this Power Moon, and when you grab it, all the lava will disappear, giving you free reign to find all nine Regional Coins (assuming you haven't already).
5. The Glass is Half Full! - Defeat Brigadier Mollusque-Lanceur III, Dauphin of Bubblaine (see above).
6. On the Cliff Overlooking the Beach - There is a high cliff near the Odyssey that you can only reach with a Gushen, and it is near the Goombas with the pirate hats. On this cliff is three Regional Coins and a Power Moon.
7. Ride the Jetstream - There is a hidden alcove in the cliff wall near the Binoculars with a pool of water in the ground, and if you head there with a Gushen, you can go up and forward to find a hidden Power Moon.
22. Treasure Trap Hidden in the Inlet - Near the Beach House, there is a small hole you can dive into that leads to four treasure chests. Open them up, left, right, then down to earn this Power Moon. The order you open the chests in is, interestingly, a reference to the "Treasure in the Ocean Cave" Power Star in Jolly Roger Bay in Super Mario 64.
24. Sea Gardening: Canyon Seed - This is the Power Moon you get for planting the seed you found in the Rolling Canyon area, which can be found here.
39. Looking Back in the Dark Waterway - Upon entering the underwater tunnel to the lighthouse, there is a wall with a big hole with it up ahead. If you go left upon passing through it, you'll find this Power Moon. You're probably expected to take a Cheep-Cheep, go through the whole tunnel, then swim through it again in the other direction to find this one, hence the name.
45. Fly Through the Narrow Valley - There is a Hat Door that you can find in the hidden alcove in the cliff wall near the Beach Volleyball course, and you'll need a Gushen to get to the door. You'll need to use the Gushens to fly through a huge chasm and over a cavern full of poisonous goop to fall down the hole leading to this Power Moon.
46. Treasure Chest in the Narrow Valley - Past the hole where you find Power Moon #45, there is a chest on a platform that you can open for a harder-to-get-to Power Moon. You'll need to be efficient with the limited water that you have with a Gushen, and you'll definitely want to get a fresh Gushen at the last point where you can get one in this Hat Door area to guarantee yourself the means to get it.