The Merman debut in the original The Legend of Zelda, they were called a Zora. They would usually pop they're head out of the water from the lake of the bodies of water around Death Mountain in Hyrule, and will shoot spit at Link.
In The Game
The Merman behaves the same way it did the original The Legend of Zelda. They can be found in most of the bodies of water found in the dungeon of the Zeldabobo. They shoot the same spit at Abobo. Usually, two of them can be placed in any spot and pop out of the water. Abobo can still defeat them with his sword, and later on, the beef.