After you make it through Route 5 and defeat Gladion on the way, you reach Brooklet Hill.
Meet Captain Lana
Cross the wooden bridge, then go south to find a Net Ball.
Talk to the fisherman to battle Fisherman Ernest. He has a level 16 Barboach and a level 16 Goldeen. You get 512 Pokédollars for winning.
Go north and talk to Lana. If you're ready to start her trial, say "Definitely!"
Lana adds Lapras to your Ride Pager. Now you can surf in water, and you may encounter water pokémon as you surf. Lana wants you to herd the Wishiwashi downstream.
Before you continue, you can go back to the wooden bridge near the entrance of Brooklet Hill and surf south to battle Fisherman Hal. He has a level 16 Tentacool. You get 512 Pokédollars for winning.
South of Hal, you find a Rare Candy.
Go west to where you were earlier. To the south, you find some grass. Use Stoutland Search to find a hidden Elixir in the southeast part of the grass.
Continue along the path, and Lana will show you some splashes in the water. Before you get in the water, go west along the shore to find an X Sp. Atk.
You can talk to the girl nearby to battle Backpacker Mikiko. She has a level 16 Fletchling. You get 384 Pokédollars for winning.
Go east along the shore, then south past Lana, and go to the end of the pier. You can talk to the fisherman to battle Fisherman Herbert. He has a level 16 Poliwag. You get 512 Pokédollars for winning.
Now check on the water and you will be asked if you want to ride a pokémon. Say yes to get into the water on Lapras. Go to the splashing water and you find a level 17 Wishiwashi. Since you are doing an Island Trial, you're not allowed to catch pokémon right now, so just knock out the Wishiwashi.
Go to the other splashing water if you wish. It's a level 17 Dewpider. Again, you can't catch it right now, but it will be added to your Pokédex.
Go to the southwest part of the water and go onto dry land there. Go to the left to get a Revive.
There is a Poké Finder spot near where you got the Revive.
Go east from there. There is a Max Repel hidden between the two sections of grass.
Up ahead, you meet Lana again. Check on the water to get in, then go to the splashing water in the middle of the pond. It's another Wishiwashi, this time level 18.
It will start raining after this.
Go east to find another splashing spot. It's a Dewpider that you can battle if you wish.
Go southeast and you can battle Fisherman Carl. He has three level 16 Magikarp. You get 512 Pokédollars for winning.
Go south from there and get out on dry land. Follow the path to find TM57 Charge Beam.
Go back into the water and go to the southwest part of the pond. Get off on dry land there. Go to the end of this path to enter the Totem's Den.
Go north and there will be a cutscene where Lana shows you the splashing water. This time, the water is the Totem Pokémon.
Before you get into the water, go west along the sand to find a Hyper Potion.
Then get in the water and go to the splashing water in the middle. A storm will start, and you will battle the level 20 Totem Araquanid. It can call pokémon such as Dewpider and Masquerain into battle.
After you win, the trial is complete, and Lana gives you Waterium Z, which you can give to pokémon that know Water-type attacks.
Lana also gives you the Fishing Rod. You can use it at rocks under the water. Press A as soon as an exclamation mark appears. Lana also gives you 10 Dive Balls, which increase the chance of catching pokémon that you encounter while fishing.
If you wish, Lana can take you to the Pokémon Center on Route 5. If you say No, you can stay here to explore Brooklet Hill more.
Lana tells you that the next trial is Kiawe's trial, at Wela Volcano Park.
Now that you have a new Z-Crystal, the Poké Mart will add Revives and Honey to their selection.
You can optionally return to Melemele Island by ferry or by Mantine Surf and explore Melemele Sea and Kala'e Bay with Lapras.
Now go southeast toward Paniola Ranch. If you have defeated all of the trainers in Route 5, be sure to talk to the Trial Guide in the southeast part of Route 5 to battle him. Trial Guide Bronson has a level 24 Slowpoke and a level 24 Mr. Mime. His Mr. Mime is holding a Psychium Z. You get 1152 Pokédollars for winning. Bronson also gives you TM96 Nature Power.
Go south to Paniola Ranch, and keep going south through the ranch. Talk to the Sudowoodos that are blocking the path. They will run away. Then the Ultra Recon Squad appears.
You can say Yes to battle Ultra Recon Squad Dulse (in Ultra Sun) or Soliera (in Ultra Moon), who has a level 20 Poipole, a Poison-type pokémon. You get 240 Pokédollars for winning.
After the battle, talk to the boy to get a Mystic Water, which makes a pokémon's Water-type moves 20% stronger if the pokémon holds it.
Go south to enter Route 6.