It's possibly played with the same instrument that sheik plays in OoT when the melody was used to warp to lake Hylia- the Lute.
It is a blue tunic which gives you the ability to breathe underwater. It can be bought from the zora shop for 300 rupees, or you can get it for free from the king zora after defrosting him. It can only be worn as an adult.
Once Lanayru Province is clear of twilight, head to Telma's Bar in Castle Town. An amnesia-stricken Ilia will be there, with Prince Ralis and Telma. You will go on a small quest to escort the sickly prince, Ilia, and Telma to Kakariko Village. Then Queen Rutela will grant you the Zora armour.