After Lana adds Lapras to your Ride Pager, you can return to Melemele Island and explore Melemele Sea.

Explore Melemele Sea

Ride Lapras from the beach near Pokémon Research Lab to enter Melemele Sea.

In the northeast part of the area, you can battle Swimmer Yasu, who has a level 17 Wishiwashi. You get 408 Pokédollars for winning.

Near the swimmer, there is an island where you can find Soft Sand.

Southeast of that island, there is a Super Repel underwater.

Near the Super Repel, you can battle Swimmer Kalani. He has a level 17 Mareanie and a level 17 Remoraid. You get 408 Pokédollars for winning.

Go southeast, then northwest onto a small beach. Here you can battle Black Belt Francis. He has a level 18 Hawlucha. You get 576 Pokédollars for winning.

Near the Black Belt, there is a Pearl String.

West of this island, you can battle Swimmer Kelsie. She has a level 17Finneon. You get 408 Pokédollars for winning.

Farther southwest, you can battle Swimmer Shelby. She has a level 18 Corsola. You get 432 Pokédollars for winning.

Keep going west to reach the western side of Melemele Sea.

West of where you entered, you can battle Swimmer Jade. She has a level 18 Clamperl. You get 432 Pokédollars for winning.

You can see a cave entrance nearby, but it is blocked by rocks. You won't be able to access that area until you have the right pokémon in your Ride Pager.

North of those rocks, there is a small beach where you can find a hidden Rare Candy.

Farther west, you can battle Swimmer Portia. She has two level 17 Luvdiscs. You get 408 Pokédollars for winning.

There is a small beach near Portia where Stoutland can help you find a hidden Rare Candy.