Pokémon FireRed and LeafGreen/Route 20

Trainer Challengers
Swimmer(Male) Barry  Shellder Lv. 31 Cloyster Lv. 31 

Swimmer(Male) Darrin  Horsea Lv. 28 Horsea Lv. 28 Seadra Lv. 28 Horsea Lv. 28 

Swimmer(Female) Shirley  Seadra Lv. 30</li> Horsea Lv. 30</li> Seadra Lv. 30</li> </ol>

Swimmer(Female) Tiffany (found by climbing ashore; she's on the other side of the island) Seaking Lv. 35</li></ol>

Pokemon Found
(Very incomplete list)

Surfing: Tentacool (probably all)

Super Rod: Krabby (really common), Gyarados (uncommon; found a lv. 20 one)