Sea Cube is a strange construction floating in the air near Jibral Castle. Approach it with your Mechat and land on it.
Activate the teleport. Take left at the junction. Defeat the fish and take 5xMedals from a chest. go back and head north (Mega Magical Medicine along the way). Reach the end of the corridor and defeat a Kelolon to get a key. Get back to the junction and head west to get to the second level.
There's an Ancient Fossil in the chest. Go along the corridor. Defeat the Kelolons to get Phoenix Wind. Turn south. At the junction head east and get rid of more Kelolons to get a key. Get back to the junction and go west. At the end you'll see a Crystal Giant that you have to beat.
HINT: By the second warp device on this level you'll find a cool item: Tech-Crusher.
Take Grand Shadow Crystal from the chest and go east. Defeat the monsters and take the key. Go back and head north. Along the way take 5xMedals from a chest and bring down the second Crystal Giant.
Take 1000 Gold from a chest and head north, then turn east (15xMedals in a chest), then north. Clear the room of Kelolons to get Lv6-Wallus. Get back to the junction and go northwards. After getting past the flying mussels go right. At the end get the Deep Sea Key. Get back and go west, then south. At the end defeat a group of Kelolons to get Infinity Ring. Go back north, defeat the giant and go to the next level.
Boss - Azure Dragon
This cool looking boss has a lot of HP, but besides that isn't really a threat. It deals little damage and at this point his attacks shouldn't trouble you at all. Attack it using fire magic or physical blows.
After the battle, two chests appear. In them you find Black Device and Laser Barrier Parts. The former lets you deactivate black barriers, and the latter is a mechat upgrade. Go north to grab Infinity Earring, Mystery Part Deux and Lv6-Shadowus.
Before you leave Sea Cube, you should deactivate two back barriers here: in Deepest Area (wall) and on Level 3 (another wall). You'll find Renew Bracelet behind the second one.