Five IslandFireRed&LeafGreen
"The fifth Sevii island, serving as a hub for the surrounding post-game areas. Lost Cave, Resort Gorgeous, and Memorial Pillar are all reachable from here."
FIVE ISLAND MAP
Island hub connecting to the Meadow to the east, Memorial Pillar to the south, and Water Labyrinth to the west. Everything branches from the central town.
Pokemon Encounters
Hoppip bobs along at 30% while surfing, one of the few Johto catches in FRLG. Shellder at 40% on the Super Rod is the best fishing target. Seadra at 4% is the rare pull.
Old Rod1
Good Rod3
Super Rod9
Magikarp
Also found in 41 other locations
Five Island Guide
Islands One through Three unlock after you beat Blaine on Cinnabar Island. Bill takes you there as part of the story. Islands Four through Seven require beating the Elite Four first, then completing Celio's Ruby and Sapphire quest on One Island to get the Rainbow Pass.
You can physically reach all seven islands without it, but evolved Johto Pokemon only show up in the wild after you get the National Dex. That means 60 or more Pokedex entries plus completing Celio's quest. If you're only seeing Kanto species, that's why.
Old Rod only catches Magikarp. Good Rod adds Horsea, Poliwag, and Goldeen depending on the area. Super Rod is where the good stuff lives: Gyarados, Seadra, Staryu, Kingler. You get the Old Rod in Vermilion, Good Rod from the Fishing Guru south of Lavender Town, and Super Rod from his brother on Route 12.
If the encounter table shows Surfing or any Rod encounters, yes. Surf itself is HM03, found in the Secret House deep inside the Safari Zone. You also need the Soul Badge from Koga in Fuchsia City to use it outside battle.