Nitro Powder
From Final Fantasy Wiki
Nitro Powder (or TNT in the original) is an item that Nerrick the dwarf uses to blow a hole that allows your ship to access to the rest of the world's waters. It is obtained in a chest in Corneria Castle. In order to get it, you must first obtain the Key from the Elf Prince in Elfheim, then head to the castle in Cornelia, where you will find the Nitro Powder in a locked room on the base floor.
