Go underwater in Inferia to 38, 8 and enter. Go down the ladder in the corner and go in the right door to be in a place with three more doors. The nearest has a Cute Mitten, the next is a Fake, and the last is a Bulkhead Key. Go back to the first three-door room and go down the middle door. Examine the device to open the bulkhead and go through. Get the Pump Room Key and go back and take the left door. Go in the first door here for a Twinkle Flute. The second has a WakeUp Charm. Continue onto the next door for a little scene, then finally exit the corridor to the right. Examine the large device to activate it.
Now, go back to the flooded room where you got the Pump Room Key and go down the two stairs into the next room. Go along and climb the ladder. Go out tthis corridor through the top door, get the Rune Bottle and go up. Watch the scene then get the chests for a Rare Plate and the Insea Map. Go down the ladder, then down the one near it. Grab the Vorpal Sword and go through the door. Go all the way to the end of the corridor and fight. Grab the cash and leave.