You will receive the Devon Scuba Gear from Steven in Mossdeep City. This event will happen automatically right after you leave the Mossdeep Gym after beating Tate and Liza.
The Scuba Gear allows you to access underwater areas in ORAS, which means this is a key item that’s required to advance the story.
Mossdeep Gym Quick Puzzle Guide (Getting the Devon Scuba Gear)
To get the Devon Scuba Gear, you must obtain the Mind Badge first. The Mossdeep Gym’s puzzle consists of multiple platforms connected by dotted lines.
There are six trainers inside the Gym, but five of them can be avoided.
All trainers use Psychic Pokémon with levels ranging from 39 to 42. Ghost and Dark types with a high Attack stat can easily sweep these battles.
Here’s a brief step-by-step guide to the gym puzzle in case you need it:
Step 1: From the Gym’s entrance, head up and step on the dotted line.
Step 2: You will be taken to the left-side platform. Walk behind the Psychic trainer and press the switch on the top-left corner.
Step 3: After pressing the switch, walk down from your current position.
Then step on the dotted line on the bottom side of the platform.
Step 4: You will return to the Gym’s entrance. Once again, walk to the center of the platform, head up and step on the dotted line.
Step 5: This time you will be taken to another platform. Press the switch on the right side.
Tip: You can avoid fighting the Psychic trainer by waiting for him to face the other way. Position yourself behind the switch to stay away from his line of sight.
Step 6: Step on the dotted line in front of you to the right and head over to the next platform.
Step 7: Press the switch on the top-right corner of this platform.
There is no way to avoid the Psychic trainer ahead. He uses a level 39 Claydol and a level 41 Starmie.
Step 8: From here, head over to the left side of the platform and step on the second dot (top to bottom).
Step 9: Press the switch on the platform’s top-left corner.
Tip: Wait for the Hex Maniac to face the other way, then walk behind her to avoid a battle.
Step 10: Walk down to the platform’s bottom-left corner and step on the dotted line. Again, wait for the Hex Maniac to face the other way in order to avoid fighting her.
Step 11: You will be taken back to the platform from Step 1.
From here, walk to the bottom of the platform and step on the dotted line facing south.
Step 12: You will return to the Gym’s entrance one more time.
Again, walk to the center of the platform and step on the dotted line at the top.
Step 13: This will take you to the final platform where you will find Gym Leaders Liza & Tate.
This is a doubles battle. Liza & Tate use a level 45 Solrock and a level 45 Lunatone.
Dark, Ghost, Grass and Water-type Pokémon with a high Attack stat will have no trouble sweeping this fight.
Step 14: Defeat Liza & Tate to obtain the Mind Badge.
Step 15: Step on the teleporter to your right after you win the battle. You’ll be returned to the Gym’s entrance.
Exit the Gym to advance the story.
Step 16: A cutscene will play as soon as you step outside the Gym. The game proceeds automatically from this point.
Step 17: Following a dialogue after the cutscene, Steven will give you the Devon Scuba Gear and HM07 Dive.
With the Scuba Gear, you can now head over to Route 128 and enter the Seafloor Cavern.