In Emerald Ruby or Saphire, use dive to go underwater. When you want to return to the surface, press the "B" button and a message should come up asking you if you want to go back up. Press yes, and you will return to the surface.
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you just press b and you will go up =P
you swim out to the spot where sootpolis is and you dive under water and go around that area until you see a spot with like a cave door and you go in. then you dive up and surf the water until you reach land.
On a small island that you surf to, then go into the forest on the island, go to your left then go to your left again then go up then go to the left then go up and there should be an old man talking about some legendary pokemon. He will give it to you if you listen to all his stories.
To get the HM dive,fly to Mossdeep city if your already there go to the Pokemon center then head left 7 steps then up 6 steps then left 2 steps then up 2 (it should take you to the middle house of three) then go in the house. Steven should see you come over to you and give you the HM dive.
if you go to the route 128 and use dive on a huge dark blue spot and look around the trench there will be a sub use dive again to go up then you are there but you need Pokemon who know strength, rock smash and surf for the mazes in the cave.[Im still stuck on the 4th maze.]