Route 21 Pokémon
- Alomomola - Common, Whirlppol, All Seasons
- Frillish - Common, Surfing, All Seasons
- Jellicent - Common, Whirlpool, All Seasons
- Mantine - Rare, Whirlpool, All Seasons
- Mantyke - Common, Surfing, All Seasons
- Remoraid - Rare, Surfing, All Seasons
After a quick visit to Route 22 and a conversation with Colress you've been directed to Route 21, to the south of Humilau City. Reaching the Route is as simple as stepping into the water on the south side of Humilau City and setting sail.
Most of Route 21 consists of a watery pathway between rocks. If you take a right as you head south you'll run into two trainers. If you squeeze through the rocks on your left you can skip the two trainers. Regardless of the path you take you'll be forced to take on a third trainer to the south.
Swimmer Desiree
- Wailord, level 46
Reward: 736 P
Swimmer Kaoru
- Remoraid, level 45
- Remoraid, level 45
Reward: 720 P
Swimmer Harold
- Octillery, level 46
Reward: 736 P
The path splits in two near Harold. Both take you to the same place, though going right will skip a trainer. Head south if you want to fight with a Black Belt.
Black Belt Kenneth
- Crustle, level 47
Reward: 1,504 P
Both paths will bring you to an open area with some shallows. Sitting on the shallows is a Big Pearl. Swimming nearby, to the right, is a trainer.
Swimmer Kyle
- Wingull, level 45
- Wingull, level 45
Reward: 720 P
Head south of Kyle and you'll find a beach. To the west you'll find the entrance of Seaside Cave, your next destination, and a Doctor is waiting outside to heal up your team (after a battle, of course). To the left of the Doctor you'll find another trainer, fishing away, and just south of this Fisherman is a PP Up.
Doctor Braid
- Alomomola, level 46
Reward: 2,760 P
Fisherman Kenzo
- Frillish, level 44
- Frillish, level 44
- Frillish, level 44
Reward: 1,408 P
Seaside Cave Pokémon
- Boldore - Uncommon, Walking, All Seasons
- Dewgong - Rare, Whirlpool, All Seasons
- Eelektrik - Rare, Walking, All Seasons - Basement only
- Excadrill - Common, Dust Cloud, All Seasons
- Frillish - Common, Surfing, All Seasons
- Golduck - Common, Walking, All Seasons
- Jellicent - Common, Whirlpool, All Seasons
- Seel - Rare, Walking, All Seasons - Ground floor only
- Seel - Common, Surfing / Whirlpool, All Seasons
- Shuckle - Rare, Walking, All Seasons - Basement only
- Tynamo - Rare, Walking, All Seasons - Ground floor only
- Woobat - Common, Walking, All Seasons
Ground Floor
If you explored Undella Bay earlier in the game there's a good chance you poked your head into Seaside Cave. This time you appear at the north end of the cave, and there's no trainer blocking your path. Time to explore. Head south from the entrance and you'll come across a trainer.
Battle Girl Tina
- Heracross, level 47
Reward: 1,504 P
Go down the stairs near Tina to find a Full Restore. From here you can head either east or west. Take the western route and jump down the ledge you find to encounter a trainer. Sitting almost beside her is a hidden Blue Shard.
Pokémon Ranger Mikiko
- Vibrava, level 46
- Gligar, level 46
Reward: 2,760 P, Persim Berry
The path splits again in front of Mikiko. Hop down the southbound ledges to find a Dive Ball and another trainer.
Pokémon Ranger Johan
- Onix, level 46
- Lairon, level 46
Reward: 2,760 P, Persim Berry
South of Johan you'll come to the trainer who was blocking the path to Undella Bay with his Roggenrola, and, yes, he's still blocking the path - though this time he'll accept your challenge. Defeat him to move his little team of rocky roadblocks, freeing up the path to Undella Bay.
Black Belt Rocky
- Roggenrola, level 44
- Roggenrola, level 44
- Roggenrola, level 44
- Roggenrola, level 44
Reward: 1,408 P
Loop back to the northern entrance of Seaside Cave using the bridge to the west of Rocky and hop down the ledge to the south. This time you should hop down the ledge on your right. This path leads to a waterway, and a trainer awaits you.
Black Belt Drago
- Scrafty, level 47
Reward: 1,504 P
Cross the water near Drago to find a Heart Scale, then Surf north. You'll debark near two sets of stairs. The one on your right is blocked by a Crustle, and you can use the Colress Machine to goad it into a battle. Catch it or defeat it, either way, you'll remove the Crustle from your path. Through the door it was blocking you'll find the end of Seaside Cave...
... though there's still more to see, so don't exit just yet. Check the rock just north of the Crustle's former spot for a Calcium, then go down the stairs on the left to find Seaside Cave's basement.
Basement
This area is full of rocks you can push around with Strength. Start by heading south until you hit the southwest corner of the area. Walk right and you'll find a Max Repel. Push the rock north of the Max Repel into the hole on its right to create a path east, where you'll find a trainer.
Battle Girl Maki
- Mienfoo, level 47
Reward: 1,504 P
Maki is standing beside a small maze of rocks. This bit looks more complicated than it seems, and once you start pushing rocks it'll be pretty obvious where each of them needs to go to let you get through the maze. Snag the Rare Candy on the right side of the maze, then head to the north end to find stairs back to the ground floor - and a trainer.
Black Belt Rich
- Gurdurr, level 47
Reward: 1,504 P
The stairs near Rich lead to a long ledge that will drop you back to the spot where you fought the Crustle. Go up the stairs on your left first and you'll find TM06 Toxic. You'll also find a ledge that will take you back to the Route 21 exit. If you're low on restorative items you may want to head back to Humilau City to stock up, as the next area has a lot of trainers. Otherwise, head through the exit Crustle was guarding.
You'll wind up back on Route 21, and the Plasma Frigate will be waiting. Hugh will show up, and after Marlon appears to open the ship your rival will charge aboard with reckless abandon. Guess you'll need to follow Hugh and back him up.