This Seeking Tech Guide will show you the fastest way to learn Water Harmony. This dance ability is only available in the World of Balance and can be easily missed.
To get to it, you first need to recruit Mog and then put him in your party. Afterwards, fly over to the Crescent Mountain Cave.
From there, jump down to the Serpent Trench again. During the following battles, Mog should be able to learn the Water Harmony dance.
However, you still need to find your way back to the airship. In order to do so, complete the rest of the Serpent Trench dungeon.When you arrive at Nikeah, you can get a Chocobo if you want. Otherwise, exit the town. From Nikeah, you have to make the long trek to Baren Falls.
Once you get to Baren Falls, jump down the waterfall. Afterwards, go southeast to head back to your airship.