After completing Acerola's trial in the abandoned Thrifty Megamart, go back to Aether House in Route 15.
Revisit Aether House
Next, you should go back to Aether House. When you get there, you have to battle Team Skull Admin Plumeria. She has a level 37 Golbat and a level 38 Salazzle. You get 4560 Pokédollars for winning.
Afterward, Plumeria says you have to go to Po Town to face Team Skull by yourself. To get there, Acerola says to look for a guy on Route 15 who is wearing a kimono.
You get a Rare Candy.
Explore Route 15
Go west from Aether House and you have to battle Ace Trainer Yuki. He has a level 34 Gible and a level 35 Marowak. You get 2380 Pokédollars for winning.
Then go north through the grass to find a Hyper Potion.
Go down the ramp near the Ace Trainer. The man in the kimono that Acerola mentioned is here on the beach, so talk to him. He will ask you what the result of his coin toss will be. Say Neither. He will add Sharpedo to your Ride Pager. This will allow you to break through boulders in the water. He says that Po Town is at the end of Route 16, which is north of Route 15.
Be aware that when riding Sharpedo, if you swim near any bubbling fishing spots, you will scare the bubbles away. You have to ride Lapras if you want to fish at bubbling spots.
With Sharpedo, you can go back to the west side of Melemele Sea on Melemele Island and break through the boulders in the water there. This gives you access to a cave where you will find TM64 Explosion.
From the beach in Route 15 where you spoke with the man in the kimono, you can ride Sharpedo north to battle Swimmer Kyra. She has a level 35 Seaking and a level 35 Lumineon. You get 840 Pokédollars for winning.
From there, go south and break through the boulders there, then go south, west, then north.
There is a narrow island to the right. On this island, you can battle Surfer Jennis. He has a level 36 Mantine. You get 864 Pokédollars for winning.
North of the Surfer, there is a building, and southwest of that building, you can get TM62 Acrobatics.
Go south to get back into the water. Go north and you can battle Swimmer Jared. He has a level 35 Tentacruel. You get 840 Pokédollars for winning.
Go northeast and you will find a beach. There, you can have a double battle with Swimmers Yumi and Jake. They have a level 35 male Frillish and a level 35 female Frillish. You get 1680 Pokédollars for winning.
North of the swimmers, there is a Dive Ball.
Get back in the water and go west. You can battle Swimmer Alexandria. She has a level 36 Bruxish. You get 864 Pokédollars for winning.
Go onto the large beach above you. There is a hidden Normal Gem on the far left side of the beach.
Near the middle of the beach, you can battle Ace Trainer Karla. She has a level 36 Alolan Sandshrew, a level 37 Wigglytuff, and a level 36 Haunter. You get 2516 Pokédollars for winning.
There is a hidden Pearl near the rightmost side of the beach.
Approach the fisherman and his pokémon to see a cutscene. Some of the pokémon that can learn Tickle are Tangela, Omanyte, Aipom, Whiscash, Mime Jr., Minccino, and Gothorita.
Then go north up the slope to reach the Route 16 Pokémon Center.