After you defeat Nessa and win the Water Badge, you need to find Gym Leader Kabu in Galar Mine No. 2.
How to Reach the Mine
To reach Galar Mine No. 2, you have to win the Water Badge from the Hulbury Stadium, then meet Chairman Rose at Hulbury's seafood restaurant. After that, you can go east from the restaurant to enter Galar Mine No. 2.
Explore the Mine
The pokémon that you may see in this area are Binacle, Chewtle, Croagunk, Noibat, Scraggy, Shellos (East Sea form), Shuckle, and Wimpod. The pokémon that you can encounter by fishing in this area are Barboach, Chewtle, and Corphish. Wimpod will run away quickly, but you may be able to catch up to it on your Rotom Bike.
When you go southeast in the cave, there is a cutscene. If you are ready, you can agree to battle Bede, who has a level 21 Solosis, a level 22 Gothita, a level 23 Hatenna, and a level 22 Galarian Ponyta. You get 2760 Pokédollars for winning.
Afterward, you get Bede's League Card.
South of where you battled Bede, you see what looks like a pokéball in the ground. This is actually a Galarian Stunfisk that you can encounter and try to catch. Stunfisk has a low catch rate, so you might want to go to stock up on Great Balls before you try to catch one. The Stunfisk in the ground in this mine will reappear if you leave the mine, so you can keep trying if needed.
Go east from this first Stunfisk and you will battle Worker Francis, who has a level 21 Carkol. You get 2100 Pokédollars for winning.
East of the Worker, you find 3 Dusk Balls, which work well in caves and at night.
Follow the path south and you can battle Worker Yvonne, who has a level 20 Roggenrola and a level 21 Timburr. You get 2100 Pokédollars for winning.
West of Yvonne, you find a Grip Claw.
South of that, you find Team Yell blocking the way. Hop will appear. If you are ready, you can have a double battle against the two Team Yell Grunts with Hop. One of the grunts has a level 22 Galarian Linoone and a level 21 Pancham, and the other has a level 21 Thievul and a level 22 Liepard. You get 3440 Pokédollars for winning.
To the west of where you battled the grunts, there is a Dreadnaw. Be sure to try to catch it.
West of that, there is a sparkle near the mine cart and barrel. Check on it to get a Star Piece.
To the south, the path splits. If you go west, there is another Stunfisk in the ground, and to the east, you can battle Rail Staff Vincent, who has a level 22 Drilbur and a level 23 Onix. You get 2760 Pokédollars for winning.
East of Vincent, you find TM49 Sand Tomb.
Go south from Vincent and the path will lead you to Hop. There will be a cutscene.
Afterward, you will go outside to the Motostoke Outskirts.