Use Dive.
You can't, You get dive from Steven in his house in Mossdeep.
Tentacool, Relicanth.
Steven will give it to you
i am rakshan.go to mossdeep city.then go to steves house.he will give you dive.
in the under water use dive
You go to the man north of mossdeep.
most water Pokemon
Dive is not an HM in firered if you want a Pokemon to learn dive trade it to ruby, sapphire or emerald then take it back after teaching it the move.
You can't get dive in fire red, however it is HM 8 in Ruby, Sapphire and Emerald.
Steven gives you the HM containing Dive he lives in mossdeep city.
You obtain HM Dive in Mossdeep City from Steven after getting the Mind Badge.