The Zora Tunic is Link's blue outfit in The Legend of Zelda: Ocarina of Time. It basically gives Link the ability to breathe underwater. Which is practically mandatory to get through the water temple.
But you could also explore lakes and other bodies of water. Like finding a piece of heart, and that's pretty much it. But it was pretty fun to just walk underwater. Which is something never works well in real life. I kinda wished there was more than just Lake Hylia and some of the Zora areas to use it with.
I also liked the color of the suit. I always picked blue (given the option) in the Super Smash Bros. games and Soul Calibur II.