As you come from the south-eastern cave, you'll arrive at the Cinnabar Island side of Route 20. Immediately to the right is a Picnicker. To her right is another Heart Scale, but this time in plain sight. If you want, you can swim across the water to get past her on the right.
Irene's Marowak can use Bonemerang for STAB, or Thrash to inflict heavy damage, but at the risk of causing confusion to itself. Stick to super-effective Grass, Water or Ice-types and you should be good.
Next can be Gloom. In case you haven't committed it to memory, this evolved Oddish is still weak to Fire, Flying, Ice and Poison. Irene's can use Mega Drain to steal HP and Moonblast to hurt Fighting and Dragon-types.
Last up is Wigglytuff, which is an evolved Jigglypuff. It's still weak to Poison and Steel. This one can use Body Slam to potentially cause Paralyse, and Mimic to clone your last move.
Jump into the water and travel west. As you're about to leave the the west side of Seafoam Islands, there's a female swimmer bobbing up and down.
Hansol has an evolved Slowpoke that boasts impressive Defence. Its weaknesses are Grass, Electric, Bug, Ghost and Dark. Be wary of its STAB Surf and Psychic, plus Yawn to make your Pokémon drowsy.
Keep going west. After a short distance, there's a small island halfway up the route. On the island is a lonely trainer and 3 Ultra Balls.
Bird Keeper Roger
Roger's brought out the big guns! These are all fully evolved Flying-type Pokémon! Hopefully you have some Electric, Rock or Ice-types handy.
First up is Fearow, which is an evolved Spearow. This Fearow can use STAB Drill Peck, and Quick Attack to go first. Electric and Rock-types should be cautious of its Drill Run. Use Ice-types if you can.
Next can be Pidgeot, a fully evolved Pidgey. Roger's Pidgeot can use Air Slash for STAB, and it also knows Quick Attack. This time, Ice-types should be careful because Pidgeot can retaliate with Heat Wave.
Finally, Dodrio is an evolved Doduo. Just like the other two, it can use Quick Attack to sneak an attack in before you. Plus it has Drill Peck for STAB, and Jump Kick to smash Rock and Ice-types.
A little ways north from this island, there's a male swimmer floating around. Go and greet him if you want.
Back to the scheduled programme. Dean's Starmie is fairly straightforward, with STAB Scald and Psychic. The best way to combat it is by using Electric, Grass, Bug, Ghost or Dark-types. Grass/Poison-types should beware of its Psychic.
Afterwards, he may say out Kingler. This evolved Krabby can use Crabhammer for STAB, or Stomp to chip away at foes that resist Water. A Grass-type is ideal to weaken the damage from Crabhammer. But a good Electric-type can probably one-shot it.
Finally, Tentacruel is an evolved Tentacool. This sinister jellyfish is still weak to Electric, Ground and Psychic. It can use STAB Surf and Poison Jab. Ground-types should be avoided unless you really believe in them.
Carry on west from the small island where the Bird Keeper was. A short distance away and a little bit south is an even smaller island. On it is a Picnicker.
This Electric-type Eevee has incredible Speed. Its most notable attack is Thunderbolt, although it can use Pin Missile, a multi-hitting Bug-type move. Ground-types are encouraged so you don't get shocked.
Next, there's a female swimmer a short distance away from the west of this tiny island.
If you haven't seen an evolved Shellder before, this is it. It has exceptional Defence, but abysmal Special Defence. Nora's Cloyster seems to be poorly trained as its best move is Water Gun. Did she use a Water Stone too early? Slap it silly with Grass or Electric-types.
Next, head north from Nora to reach a third island. Here, you can pick up a Max Repel to the right. Hop back into the sea and head directly west. There will be another female swimmer not too far away.
Seaking shouldn't be a surprise. Melissa's can use Waterfall for STAB, Quick Attack to try and attack first, and Megahorn to deal Bug-type damage. If you have a Grass/Poison, you don't need to worry much, as Megahorn will inflict neutral damage. Otherwise, choose an Electric-type if possible.
Next is Jynx, which isn't a Water-type, but close enough we guess. This charming lady is weak to Fire, Rock, Bug, Ghost, Dark and Steel. We'd be surprised if you didn't have one of those. For attacks, it can use STAB Ice Beam and Psychic, plus Lovely Kiss to inflict Sleep.
Moving on, there's one more island to Melissa's south. In the middle is an Ace Trainer. If you search the left side, you can find some hidden Stardust too.
Ace Trainer Marty
Golem is a fully evolved Geodude. This rock-hard beast is still weak to Ground, Fighting and Steel, and immensely weak to Grass and Water. For attacks, it can use STAB Rock Slide and Earthquake. Because of the latter, avoid fielding Steel-types.
Marty's second Pokémon is Pinsir, which is a pure Bug-type. This one can use STAB X-Scissor, plus Superpower to hurt Rock-types etc. The latter will reduce Pinsir's Attack and Defence, so it's only threatening the first few times. We suggest sending out a Fire or Flying-type.
Finally, carry on west for a wee bit longer and you'll come into Cinnabar Island. Now you can relax for a while. All that surfing and cave exploring must have tired you out.