After you meet Nessa at the Lighthouse, you enter Hulbury Stadium to try to win the Water Badge.
Get a Lure Ball
In Hulbury Stadium, talk to the Ball Guy to get a Lure Ball.
Hulbury Stadium Gym Mission
Talk to the person at the north end of the stadium to begin the Hulbury Gym Mission.
From the starting area, go east and you will battle Gym Trainer Julia, who has a level 21 Tympole. You get 2016 Pokédollars for winning.
Go east from the trainer and press the red button, then go north and press the yellow button, then go south and press the red button again.
Go east, then north. Ignore the red button and go north, then west, to battle Gym Trainer Heather, who has a level 20 Krabby and a level 20 Corphish. You get 1920 Pokédollars for winning.
Go north from the trainer and push the yellow button, then go south and push the red button. Go west from there, then north up the stairs, then go west and follow the path. It leads to Gym Trainer Lynn, who has a level 20 Remoraid and a level 21 Chewtle. You get 2016 Pokédollars for winning.
Go south from the trainer and push the yellow button, then go northwest and push the red button, then go east and push the blue button. Then go east from the trainer, and you can go north up the middle of the gym to reach the door to the Gym Leader.
Gym Leader Nessa
Nessa has a level 22 Goldeen, a level 23 Arrokuda, and a level 24 Drednaw. As before, you can Dynamax your pokémon by pressing the Dynamax button to the left of your pokémon's moves. Your pokémon will stay Dynamaxed for 3 turns. You get 3840 Pokédollars for winning.
You get the Water Badge, which allows you to catch pokémon up to level 30. You also get TM36 Whirlpool, and a Water Uniform.
You can now buy Poké Dolls at Poké Marts.
Meet the Chairman
When you leave the stadium, Oleana reminds you to meet the Chairman at the seafood restaurant, the Captain's Table. She gives you Rose's League Card.
Go east from the Hulbury Pokémon Center to find the restaurant. Go inside and approach Sonia, and there will be a cutscene. You will get TM79 Retaliate.
Afterward, go outside and there will be a cutscene. To proceed with your Gym Challenge, you will need to go east down the road from the seafood restaurant, where you will find the entrance to Galar Mine No. 2.