• Another dungeon underwater, see the map below where it is located.
• All doors in the dungeon are locked. So, move your Thief to the top (Inventory -> Formation) so that he/she can open the locked doors.
• Follow the map, collect all chests.
• After you obtained Noah's Lute, you can either you Teleport out or backtrack to get out.

Chests (10):

• Diamond Helm
• Diamond Bracers
• Defender
• Diamond Mail
• Diamond Gloves
• Behemoth Knife
• Blood Lance
• Protect Ring
• Lamia Harp
• Diamond Shield

Now go to Unei's Shrine, talk to her, and watch a scene. She will tell you that you need a new airship, Invincible.