Hoenn walkthrough section
Pokemon Ruby and Sapphire Phase 7: Slateport Harbor
Mind Badge Fly to Slateport and go to the harbor. Captain Stern is being interviewed about the submarine.
Phase 7 Mind Badge Lilycove, Mt. Pyre, villain hideouts, Mossdeep, Tate and Liza, and Dive. 07 / 10 
Phase 7 section 4 of 8
Fly to Slateport and go to the harbor. Captain Stern is being interviewed about the submarine.
GuidePokemon Ruby and Sapphire
PhasePhase 7
Section4/8
Slateport Harbor
Fly to Slateport and go to the harbor. Captain Stern is being interviewed about the submarine. Your villain team interrupts the scene and steals it, which unlocks the Lilycove hideout as the next required dungeon.
This is one of the easiest story beats to miss if you fly straight back to Lilycove after Mt. Pyre. Mt. Pyre -> Slateport Harbor -> Lilycove Hideout is the correct chain.