You need to find your way to Sootopolis. That’s where the 8th gym (Water-type) will be. Getting to Sootopolis can be a bit confusing. You’ll need Surf and Dive. when you reach an extra dark spot in the water—a trench—use Dive to go down, then navigate through the water until you find a huge underwater cave. Go through that cave, swim to the light spot, use Dive again, and you’ll pop up in Sootopolis.
GET THE LAST BADGE......
With the seventh badge in Pokemon Ruby version, you can control caught or traded Pokemon up to level 70.
The seventh gym badge in Emerald is located in Mossdeep City.
at the gym
You go to get your 7th Badge in Mossdeep City.
Go to the Pokemon league.
the ground gym
Dewford Town.
Destroys picachew
the seventh gym badge snowpoint gym
when you get your seventh badge and you will head for mount cornet
Beat the gym leader