Walkthrough
Route 19 connects to Route 20, a much longer sea route. Route 20 flows directly into Cinnabar Island; however, it is separated by the Seafoam Islands. Believe it or not, those islands will actually make it impossible to get from one side of Route 20 to the other without first going through one section of them. Loads of Wild Pokemon and Trainers can be found across both sides of the enormous Route 20.
Wild Pokemon
Waters:
Tentacool (45%): Levels 37-44
Magikarp (45%): Levels 37-44
Tentacruel (9%): Levels 37-44
Gyarados (1%): Levels 37-44
Waters - Special Spawns:
Lapras: Levels 37-42
Sky:
Pidgey (60%): Levels 3-56
Pidgeotto (35%): Levels 3-56
Pidgeot (5%): Levels 3-56
Sky - Special Spawns:
Dragonite: Levels 3-56
Charizard: Levels 3-56
Articuno: Levels 3-56
Zapdos: Levels 3-56
Moltres: Levels 3-56
Trainers
Swimmer Darrin: Level 44 Golduck, Level 44 Gyarados
Beauty Shirley: Level 41 Wartortle, Level 41 Starmie
Picnicker Irene: Level 43 Marowak, Level 43 Gloom, Level 43 Wigglytuff
Beauty Hansol: Level 43 Slowbro
Bird Keeper Roger: Level 45 Fearow, Level 45 Dodrio, Level 45 Pidgeot
Swimmer Dean: Level 44 Starmie, Level 44 Tentacruel, Level 44 Kingler
Picnicker Missy: Level 41 Jolteon
Beauty Nora: Level 41 Cloyster
Ace Trainer Marty: Level 45 Golem, Level 46 Pinsir
Beauty Melissa: Level 43 Seaking, Level 43 Jynx
Obtainable Items
Poke Balls: Defeat Swimmer Darrin
Poke Balls: Defeat Beauty Shirley
Great Balls: Defeat Picnicker Irene
Heart Scale: Near Picnicker Irene
Poke Balls: Defeat Beauty Hansol
Ultra Balls: Near Bird Keeper Roger
Poke Balls: Defeat Bird Keeper Roger
Poke Balls: Defeat Swimmer Dean
Great Balls: Defeat Picnicker Missy
Poke Balls: Defeat Beauty Nora
Max Repel: Near Beauty Nora
Ultra Balls: Defeat Ace Trainer Marty
Poke Balls: Defeat Beauty Melissa
Post Game
Walkthrough
After defeating the Elite Four and starting the Master Trainers Challenge, you will find a number of Master Trainers on Route 20. In total, there will be five Master Trainers to defeat.
Trainers
Swimmer Tessa (Master Trainer): Level 80 Blastoise
Scientist Francis (Master Trainer): Level 75 Tentacool
Swimmer Prita (Master Trainer): Level 70 Wartortle
Scientist Melvin (Master Trainer): Level 80 Tentacruel
Swimmer Rose (Master Trainer): Level 75 Cloyster