You’ll follow the path to the bottom of the screen. You’ll see a large rock formation in the center of your screen with some braille on the south-end. When you interact with the braille sign, you’ll be prompted to dive again with Wailord. You’ll pop up in a chamber not filled with water.
You’ll probably get a pop-up that says something like “Want to escape from here and return to Underwater?” Make sure you respond with “Yes” to continue.
Run north again to the next braille sign and press A to read it. If you have Wailord at the front of your party and Relicanth at the end, an earthquake should be triggered and a door will open at each of the Regi sites. You’ll definitely want to save here so you don’t have to open the doors again.
Go over to the braille sign in the north end of the cave and press A to read it. Then go two steps left, then two steps down and use Rock Smash. You’ll want to save here before battling, just in case you don’t catch Regirock, so you can have another chance.
Go over to the braille sign in the north end of the cave and press A to read it. Then go two steps left, then two steps down and use Rock Smash. You’ll want to save here before battling, just in case you don’t catch Regirock, so you can have another chance.
Go over to the braille sign on the north wall and press A to read it. Staying next to the walls at all times, do a counterclockwise lap (walking, running, or on your bike) around the entire cave. Once you do, the wall with the braille sign will reveal a cave entrance for you to enter. You’ll want to save here before battling just in case you don’t catch Regice, so you can have another chance. Regice has an Ice Wind skill that will lower your Pokémon’s speed. You can use a Pokémon with Yawn to get rid of this effect.
Go over to the braille sign on the north wall and press A to read it. Go to the middle of the room and use Flash. You’ll want to save here before battling, just in case you don’t catch Registeel, so you can have another chance.