i think you get to the underwater cavern after beating the 7th gym leader
You must beat the MossDeep City gym to use the HM for Dive.
you will find team aqua in the under water cavern near the dive point in sootopolis
simple you go to mossdeep city vis surf beat tate and liza then help steven beat team magma in the space place at the northeast of the city and then go into all houses until you reach Stevens house and use dive to find the seafloor cavern and beat team aqua
just surf, surf and to the sotopolis island is dive zone dive! (there is two dive zones. one to sotopolis, second to team aqua cavern.
you cant, you use dive next to the sub and you will find them inside the cave
It is in the cave where Archie of Team Aqua tries to capture Kyogre. Alas you need HM Dive to get there.
you get it from team aqua
Steaven gives it to you at mossdeep city, go to his house after you beat the gym and he will give it to you.
You can't unless you trade a Pokemon from Pokemon Ruby, Sapphire or Emerald and get a Pokemon that knows Dive. The HM Dive is not necessary in Pokemon Firered and is thus not avaliable. However, a Pokemon that knows Dive can be traded from Pokemon Sapphire, Ruby or Emerald version to Firered.can notI can't remember much, but I think you have to get it from Wallace or Juan.
go to sootopolis city but the door is locked to the gym so fly to mossdeep city surf down allitle there will be dark water use dive beat team aqua/magma and beat or catch kyogre/groudon and youll be able to go in the gym.Go south of Mossdeep to where the water is darker and use dive there. Keep going south underwater and you will find the entrance to the Team Aqua hideout. You have to defeat Archie, the Team Aqua leader.
after you beat mossdeep gym talk to steven go to steven's house in moss deep town
there a dive area and once you dive there, there youll find the submarine but you really need to look. the entrabce is like entering a cave underwater