Walkthrough
Just east of the Six Island main area, you will find the Water Path. As the name might suggest, this path is mainly made up of water. Loads of Wild Pokemon and Pokemon Trainers can be found here.
Wild Pokemon
Grasses:
Sentret (30%): Levels 10-15
Spearow (20%): Level 44
Fearow (15%): Levels 48-50
Oddish(FireRed)/Bellsprout(LeafGreen) (10%): Level 44
Meowth (10%): Level 41
Gloom(FireRed)/Weepinbell(LeafGreen) (5%): Level 48
Persian (5%): Levels 47-50
Psyduck(FireRed)/Slowpoke(LeafGreen) (5%): Level 41
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
Hiker Earl: Level 49 Onix, Level 49 Machoke
Juggler Edward: Level 46 Voltorb, Level 46 Voltorb, Level 47 Electrode, Level 48 Mr. Mime
Swimmer Denise: Level 49 Chinchou, Level 49 Lanturn
Swimmer Samir: Level 50 Gyarados
Twins Miu & Mia: Level 50 Pikachu, Level 50 Pikachu
Aroma Lady Rose: Level 49 Sunkern, Level 59 Sunflora
Pokemon Trainers - VS Seeker Rematch
Hiker Earl: Level 54 Onix, Level 54 Machamp
Swimmer Samir: Level 55 Gyarados
Items
Dragon Scale: On Ground
Elixir: On Ground
Pinap Berry: ItemFinder
Aspear Berry: ItemFinder
Oran Berry: ItemFinder
Nest Ball: Break Heracross Lady's Record