Before you get onboard the ship, use Frost on the puddle, then climb up and hop across. Inspect the crate to get a Cookie. Go back to the boat, and approach the door. Once you're inside, head through to the next room.
You'll have to fight several monsters called Aqua Jellies in here - they're only a tiny bit harder than a regular enemy. Make sure you fight all of them, even if you don't have to, because you'll want the experience and money. Anyway, kill the one in your way and hop over the puddle it leaves behind. Go down the ladder and beat this Aqua Jelly, then freeze the puddle it leaves behind. Hop across and get the Potion from the chest, then head right.
Follow the path through the room with the many Aqua Jellies. In the next area, defeat all the Aqua Jellies (ignore the one you can't get to). Shove the large box into the bottom right of the square, then use Frost on the bottom two puddles to get across. In the corridor, shove the box off and enter the door to the right.
This is a big room - start by beating the Aqua Jelly down the ladder to your right. Freeze its puddle and then go up and hop across the ice pillar and beat the next Aqua Jelly on the other side. Freeze this puddle, then go up to the next Jelly, beat it, and freeze it. Head back to the entrance of the room and take the north ladder this time. Beat and freeze this Jelly, and then use the pillar to hop across and beat the next one. Freeze this puddle as well, then Move the large Box so that the pillars form a path to the next platform. Go back up and use this path. At the top of the room, go down the ladder, beat and freeze the Jelly, and then use the ice pillar to exit the room through the door to the left. Back in the room you were in before, go down the ladder and beat the Jelly. If you try to walk across the bridge it falls, so freeze the Jelly to stop the bridge from moving. In the Jelly-filled room, you can only approach one of them. But before you do, set Felix's Mercury Djinni and Iron, set all of Jenna's Mars Djinn, set Breath and Ether of Sheba's and all of Piers' Djinn except Shade. Heal if necessary, then try to attack the Jelly in the middle.
Boss Battle: Aqua Hydra
All of the Jellies in this room will combine to form this massive thing. Don't worry though, it's quite easy to beat. The first turn should consist of Ragnarok from Felix, Meteor from Jenna, Flora from Sheba and Boreas from Piers. Next turn, a series of powerful Psynergies along with Piers unleashing Shade and Felix unleashing Echo is in order. It'll be down to half health anyway from your first turn.
Next turn, Steel, Char, Blitz, Fog. It'll be weakening quite quickly now. Once it's your go again, summon Ramses, Magaera, unleash Waft and summon Nereid. It shouldn't be living by the end of this turn. You'll get a vial for beating it, but now you can hop across the boxes to get to the left-hand door. Follow the path to go down a deck.
Down here, move the pillar over the drop symbol on the floor, then use Douse on the pillar. This will open the door at the top. Follow the path around to the next door, and you'll be at the core. Soon, you'll be back on top deck, looking out at the ocean. Everyone will want to go to Vale and places, but you can't, for several reasons. You can go exploring pretty much anywhere, but now you can go back to the Gabomba Statue in Kimbombo and do Gabomba Catacombs. You don't need to go back via land - you can sail around to a beach nearby. So, go in-between Indra and Gondowan. Ignore the island for the minute and land on the beach just to the west of it. Go back across to the north of Kimbombo Mountains, and head back up to Kimbombo.
Back here, use whatever you need to, then enter the Great Gabomba via the tongue. Go back to that final chamber, and descend the ladder into Gabomba Catacombs.