I don't remember how, I already cleared Magma Hideout (I think), the cave in Lilycove city in the sea have members of team Aqua blocking the entrance, I don't know what I need make more...
Do you mean the guys blocking the water route with Wailord? If so, go to Slateport City. Go to the building near the Pokemart(I think, if they aren't there check some other buildings). There should be a bunch of Aqua Grunts in front of it. Investigate to unlock the next mission. Source: my memory of the Game Guide.
