From Fuchsia City, you can go south to Route 19, which leads to Route 20. From there you can go west to the Seafoam Islands, where a rare pokémon awaits. You can also access Route 20 by going east from Cinnabar Island.

Eastern Route 20

If you arrived in Route 20 from Route 19, then in the northeast part of Route 20, you can battle Swimmer Douglas, who has a level 42 Gyarados. You get 840 Pokédollars and 3 Poké Balls for winning.

South of there, you can battle Swimmer Darrin, who has a level 44 Golduck and a level 44 Gyarados. You get 840 Pokédollars and 3 Poké Balls for winning.

Go northwest from there, then west, and you can battle Beauty Shirley, who has a level 41 Wartortle and a level 41 Starmie. You get 820 Pokédollars and 3 Poké Balls for winning.

Go west, then go onto the land. Go west and you find the entrance to the Seafoam Islands. Go west from there and talk to the girl to heal up your pokémon.

Then go into Seafoam Islands when you are ready.

Western Route 20

If you arrived in Route 20 by going east from Cinnabar Island, go east and you can battle Beauty Melissa, who has a level 43 Seaking and a level 43 Jynx. You get 860 Pokédollars and 3 Poké Balls for winning.

South of the Beauty, there is an island. Check the middle of the island to find a hidden Stardust.

On this island, you can battle Ace Trainer Marty, who has a level 45 Golem and a level 46 Pinsir. You get 4600 Pokédollars and 5 Ultra Balls for winning.

To the northeast, there is an island where you find a Max Repel.

South of that island, you can battle Beauty Nora, who has a level 41 Cloyster. You get 820 Pokédollars and 3 Poké Balls for winning.

East of the Beauty, there is an island where you can battle Picnicker Missy, who has a level 41 Jolteon. You get 820 Pokédollars and 2 Great Balls for winning.

To the northeast, you can battle Swimmer Dean, who has a level 44 Starmie, a level 44 Tentacruel, and a level 44 Kingler. You get 880 Pokédollars and 3 Poké Balls for winning.

Go south to find an island where you can battle Bird Keeper Roger. He has a level 45 Fearow, a level 45 Dodrio, and a level 45 Pidgeot. You get 2700 Pokédollars and 3 Poké Balls for winning.

On this island, there is a set of 3 Ultra Balls.

Go southeast from there and you can battle Beauty Hansol, who has a level 43 Slowbro. You get 860 Pokédollars and 3 Poké Balls for winning.

Go east, then north onto land. Go east and you can battle Picnicker Irene, who has a level 43 Marowak, a level 43 Gloom, and a level 43 Wigglytuff. You get 860 Pokédollars and 2 Great Balls for winning.

East of the Picnicker, there is a Heart Scale.

Go into Seafoam Islands when you are ready.