I Think when you surf in the water i dont know.I think
Chat with our AI personalities
You can't get a Tentacruel by just fishing/surfing in Pokemon FireRed. You have to Evolve a Tentacool (It evolves at level 35).
Tentacool evolves on Lvl. 30 into Tentacruel.
Tentacool evolves at level 30 into Tentacruel.30
Use Surf on Route 205 and you'll easily find a Tentacool.
In Pokemon firered you can't, but in Pokemon leafgreen you can get it then trade it to Pokemon firered.