Be careful of the Ghost Ship though. He's hard to find and when you finally get a battle with it, don't kill it right off. I suggest equiping a sense materia to keep track of his HP, then use morph, when he's getting low. If you do a load of damage right off, it'll use Gonnai, an attack that will knock a character out of battle, like Midgar Zolom. Do it right, and you should have an Underwater Materia soon.