Final Fantasy V/Submarine

Gill Shrine
Though the barrier is now down and you can enter X-Death's castle, the door to the main part of the castle is locked, so other than talking to Zeza's soldiers who are occupying the lower levels, there's nothing really to do there yet.

There are three points of interest marked on your underwater map, and the one in the center marks Gill Shrine. This is what is left of Gill's island since it's now underwater. Sail around X-Death's island and locate a rocky spot on the otherwise sand covered ocean floor. As soon as the sub is over it you autmatically dock.

Go south to reach B. 2 Floor. Cross the room and go through a doorway to a room with five chests in the middle. The one that's closed has a big rock in it; this holds the lid down which then holds the doorway open. You can change which of the five doorways on the north side of the room is open by moving the rock to different chests. To save time, you might as well know that the doors on the far left and far right are dead ends, and since these are controlled by the chests on the right you don't have to bother with those two at all. Take the rock out of the center chest and place in the chest in the lower left; this opens the middle right door. Go in and press the button here to open a passage in another room. Now go back and move the stone from the lower left to the upper left; this opens the middle door. Go in, follow the newly opened passage to B. 3 Floor

Around this time you start getting random battles with a monster called Metamorpha, who is really ten monsters in one since it can change to one of nine alternate forms. You have seen these forms as other monsters, some of which are easy to handle but others may be tricky if you're not prepared. Metamorpha has high ABP but low Exp so this is a good spot to develop jobs without leveling up.

Look for the dark passage in the southwest corner, follow and press the button at the end. This opens a doorway in the center of the room. Go through this doorway down the stairs ot B. 4 Floor. This floor has dark passages similar to the ones in Valley of Dragons; follow them west, south, and east, then exit to B. 5 Floor

This seems like a dead end, but when you get close to the edge of the pond here you see a turtle. Jump in to be sucked through an underwater passage and emerge in a new area. Cross the path over the water to meet the turtle again. Bartz starts to poke at it but then it asks him to stop, which gives Bartz a bit of a shock because turtles don't normally talk in his world. Galuf says the turtle is actually Sage Gill and Gill tells you that X-Death is looking for something in Moore Forest where X-Death originally came from. It seems that 500 years earlier some evil spirits trapped in the forest cast a spell on a tree and it became X-Death. X-Death was still trapped in the forest until he escaped 30 years ago. Finally Gill gives you the Elder's Branch which allows you to enter to forest.

Cast Telepo to return to the sub.

Gill mentioned that the forest, your next stop, is west of Surgate Castle.