you have to go by sootopolis city and use dive.
you would have to beat maxie in team magma and a team magma admin then you should see steven and then steven will give a beldum and dive.
maxie
i think you need to beat everyone (only team magma) then get to maxie (team magma's leader) & battle him. use level 49's & 50's to beat him
YOU TRADE IT WITH MAXIE THE TEAM MAGMA LEADER
i don't know but i remember that you can access the Magma Cave/Hideout in order to battle Maxie / or even not. I think after you defeat Maxie, it is closed. But i Already Finished Pokemon Emerald..
U go to the magma hideout and defeat Maxie
Team Magma boss Maxie in their hideout in the cave by Lilicove's shore
When you defeat Maxie on Mount Pyre in Pokémon, he is typically found in the chamber where the player battles him. After his defeat, he expresses his disappointment and reflects on his goals regarding Team Magma's plans. Following the battle, he leaves the area, allowing the player to continue their journey.
team magma boss Maxie in their hide out along the cave by lilicove's shore
Team Magma's leader's name is Maxie. There are three admins: Courtney, Tabitha, and Mack
The Leaders of Team Magma and Team Aqua.
After you beat team magma's leader, Maxie, surf to Lilycove city. Beat the gym and find hm dive.Then surf to a dark blotch in the ocean.Use hm Dive.