After having obtained and taken control of your airship, time to learn controls. Press B to land (you can only land in water!) and then press A to board.
Fly off of the edge of the Floating Continent and, after piercing the clouds, you’ll find yourself on a largely-flooded surface.
After listening to the nice music, go to the northernmost island. You’ll find a tiny chunk of forest and a ship there. Head into the ship.
Go down to the bottom deck and snatch the Blood Sword and the Zeus’s Wrath . The former is a nice, HP-draining weapon for if you need to heal and want to damage at the same time.
Speak with the old man nearby, then the woman. Hand her an Antidote to wake her up and cure her. She’ll then tell you about the Temple of Water, which sounds like where you could find the Water Crystal. Well, let’s go!
Fly over to the largest of the island here and you’ll find the Temple of the Water and the Cave of Tides there. Go inside the temple. Go along the path to the Crystal room and you’ll get a Crystal Shard . Then go into the cave.