Answer:
You'd have to have encountered Groudon in your story first. You're able to unlock Lugia as early as the battle with the 8th gym leader. First go east of Dufour town, once you landed on the ship go underwater to the control room. Pick up the scanner, and go to Captain Stern in the Stateport city ship dock. Give him the scanner, and he will give you a bell. Go back to Sea Marvel and notice your item start to resonate. There will be a portal underwater in the control room. Go through it and after that you'll see what happens, depending on which bell you have.
Step-by-step explanation:
Hope this helped!!! Good luck!- Nya~ :3