go to the dive spot surrounding a cave. (not the white cave) then, go in. see the submarine? press b here and you'll get to seafloor cavern. (make sure your Pokemon knows strengh) you'll battle some...
fly 2 slateport go 2 the dock talk to capt stern and then see team magma steal a sub fly 2 the city with the hide out and the duds r gone (while ur in the base befor u leave find a room with 4 poke...