Sol, Aigis and Dusk heads towards Burtgang to find Sarah on Dr. Lugae's battleship and they are attacked by an imperial airship. The airship is very fast so they cannot escape it on the battleship. Aigis then suggests using the Bottle of Bubbly Water to be able to breath underwater. Dr. Lugae refuses to use it as he wants to be with Argy and urges the warriors to jump off the ship. He and his ship sinks together while the others made it to an Underwater Cave. By traveling through this area, they finally get to Castle Burtgang.
Spoilers end here.
There is a pot that can fully restore HP and MP.