Walkthrough
South of the main area of One Island is Treasure Beach, only accessible by using HM Surf. Treasure Beach is home to loads of different Wild Pokemon, as well as one lone Pokemon Trainer.
Wild Pokemon
Grasses:
Spearow (30%): Levels 31-32
Tangela (30%): Levels 33-35
Fearow (20%): Levels 36-40
Meowth (10%): Level 31
Persian (5%): Levels 37-40
Psyduck(FireFred)/Slowpoke(LeafGreen) (5%): Level 31
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) (80%): 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 Amara: Level 36 Seel, Level 36 Seel, Level 36 Dewgong
Items
Ultra Ball: ItemFinder
Star Piece: ItemFinder
Ultra Ball: ItemFinder
Big Pearl: ItemFinder