Walkthrough
Just north of the Green Path is Outcast Island. As the name suggests, the area is pretty remote and thus doesn't contain all that many Pokemon Trainers and Wild Pokemon. A Team Rocket Member will be found on Outcast Island, but they won't reveal the second password to the Team Rocket Warehouse. Altering Cave can be found with the Outcast Island Area.
Wild Pokemon
Surfing:
Tentacool (95%): Levels 5-40
Tentacruel (5%): Levels 35-40
Magikarp (100%): Level 5
Horsea(FireRed)/Krabby(LeafGreen) (80%): Levels 5-15
Magikarp (20%): Levels 5-15
Horsea(FireRed)/Krabby(LeafGreen) (40%): Levels 15-25
Qwilfish(FireRed)/Remoraid(LeafGreen) (40%): Levels 15-25
Gyarados (15%): Levels 15-25
Seadra(FireRed)/Kingler(LeafGreen) (4%): Levels 25-35
Psyduck(FireRed)/Slowpoke(LeafGreen) (1%): Levels 25-35
Pokemon Trainers
Swimmer Nicole: Level 50 Marill
Sis & Bro Ava & Geb: Level 50 Poliwhirl, Level 50 Starmie
Swimmer Mymo: Level 49 Kingler, Level 49 Wartortle
Team Rocket Grunt: Level 48 Muk, Level 48 Golbat, Level 48 Raticate
Fisherman Tylor: Level 49 Qwilfish, Level 49 Qwilfish
Pokemon Trainers - VS Seeker Rematch
Swimmer Nicole: Level 54 Marill
Items
PP Up: On Ground