Walkthrough
After defeating both the Fuchsia City Gym and Saffron City Gym, it will be time to head south of Fuchsia City on Route 19. You will need HM Surf to do so since this is Water Path, which can be obtained by making it all the way through the Safari Zone in Fuchsia City. Route 19 will take you to Route 20, which will lead you to both Sea Islands (where another Legendary Pokemon can be found) and Cinnabar Island (where the seventh Pokemon Gym is located).
Wild Pokemon
Surfing:
Tentacool (100%): Levels 5-40
Magikarp (100%): Level 5
Horsea(FireRed)/Krabby(LeafGreen) (60%): Levels 5-15
Krabby(FireRed)/Horsea(LeafGreen) (20%): Levels 5-15
Magikarp (20%): Levels 5-15
Horsea(FireRed)/Krabby(LeafGreen) (84%): Levels 15-35
Gyarados (15%): Levels 15-25
Psyduck(FireRed)/Slowpoke(LeafGreen) (1%): Levels 25-35
Pokemon Trainers
Swimmer Richard: Level 30 Tentacool, Level 30 Shellder
Swimmer Reece: Level 29 Goldeen, Level 29 Horsea, Level 29 Staryu
Swimmer Tony: Level 30 Horsea, Level 30 Horsea
Swimmer David: Level 29 Goldeen, Level 29 Shellder, Level 29 Seaking
Swimmer Douglas: Level 27 Horsea, Level 27 Tentacool, Level 27 Tentacool, Level 27 Goldeen
Swimmer Matthew: Level 30 Poliwag, Level 30 Poliwhirl
Sis & Bro Lia & Luc: Level 30 Seaking, Level 30 Goldeen
Swimmer Axle: Level 27 Tentacool, Levle 27 Tentacool, Level 27 Staryu, Level 27 Horsea, Level 27 Tentacruel
Swimmer Alice: Level 30 Goldeen, Level 30 Seaking
Swimmer Connie: Level 29 Staryu, Level 29 Staryu, Level 29 Staryu
Swimmer Anya: Level 27 Poliwag, Level 27 Goldeen, Level 27 Seaking, Level 27 Goldeen, Level 27 Poliwag
Pokemon Trainers - VS Seeker Rematch
Swimmer Tony: Level 49 Seadra, Level 49 Seadra
Swimmer Matthew: Level 49 Poliwhirl, Level 49 Poliwrath
Swimmer Alice: Level 49 Seaking, Level 49 Seaking