Ocarina of Time
Link aquires the Blue Tunic (or Zora's Tunic) from King Zora in return for unfreezing him. You can also buy one in the Zora shop for a fee of 300 Rupees. This tunic allows Link to breathe and survive underwater, and gain access to the Water Temple. See Zora Armor
In Link's Awakening, the Blue Tunic serves a completely differnt purpose than in Ocarina of Time. Firstly, it is not required to complete the game and is optional to aquire. You can only get this tunic in the DX version of the game in the Color Dungeon. After completeing the dungeon, the Great Fairy will give you an option to have more power or defense. The Blue Tunic will grant you double defense, while the Red Tunic doubles your attack.