They are in Seafloor Cavern.
Things you need to get there:
Surf HM
Dive HM
First Surf directly downwards Mossdeep and then Dive at the first dark Patch you see.Follow the route until you see a hole.go through it and you will find a submarine.Surface and you are at Seafloor Cavern!
Water, dark and grass Pokemon
get a pokemon, then you train it, get it really strong, and beat th ecrap out of all the 7th leader's pokemon. Its easy.
Beat Archie
fly to lilycove and do into the cave at the noth of the beach,get loads of potions because you fight archies admin at the end
he is at the top of team aqua HQ, after you beat him you get the master ball
the cave of origin but you have to beat team aqua first
For Pokemon emerald, you have to go to the weather instinute after you beat the 7th gym leader where you beat team magma for the last time then you go to route 129 where the seafloor cavern is where you battle team aqua for the last time and that is where kygore gets awakened. then you go to sootpolis city where you talk to stevn who leads you to Wallace than you go to the sky pillar. then you go to sootpolis and battle the 8th gym leader
Mossdeep city here is how you get there. Beat team magma in their hideout and beat team aqua in their hideout to gain entry to mossedeep city. Mossedeep city is right of lilycove city
OF COURSE! You have to beat team Aqua first. [or team Magma]
You need to get past the Team Aqua grunts at Lilycove Town to advance on Pokemon Emerald. In order to do this, you first have to beat Team Aqua grunts at Lilycove Town.
You should get a Duskull and evolve it [it evolves at lvl 37 to Dusklops] then get a Shuppet the evolve that [it evolves at lvl 37 too] then teach it a ghost type move or dark type move, then train and that is how to beat the 7th gym leader in Pokemon Emerald. Good luck!!
The seventh gym leader