What to do after you get the 6th badge in pokemon sapphire?

Go to the Lilycove Dept. Store, and standing in front is your rival that you fight. Go back to Slateport. In the Northeast part of the town you will see a large group of people gathered. Walk up and talk to the professor and after all the dialog, fly back to Lilycove. Walk east to the shore there, use surf and go north. Go in the cave and the 2 people from team aqua won't be there to block you anymore.