Catfish's Maw is the fifth dungeon. This dungeon is located in the center of Martha's Bay.
This trick requires you to obtain the boomerang before getting the hookshot. By transitioning into the fourth Master Stalfos fight room with the key from the room next to it the key changes into the hookshot, allowing you to skip the second and third Master Stalfos fights.
One of the minibosses of Catfish's Maw are two Gohma's. Gohma is only vulnerable when her eye is open, only taking damage from arrows or the hookshot aimed at the eye. The ocarina can be used to stop Gohma in her tracks and open her eye shortly afterwards.
Even though Master Stalfos does not spawn a portal, Master Stalfos is another miniboss unique to Catfish's Maw. Similar to the Master Stalfos' in Link to the Past, he has to be knocked down first before you can hurt him. This can only be accomplished by using a spinslash with the sword. Once knocked down he is vulnerable to bombs and sword beams from the L-2 sword. The amount of hits necessary for him to "give up" changes depending on in which room you are fighting him. In the first room, Master Stalfos "flees" after getting hit three times, while in the second and third room he "flees" after being hit only twice. In the fourth room his death animation starts after 3 hits. In the first three rooms it is actually possible to kill Master Stalfos instead of making him flee. In order to kill him, you need to hit him 4 times. This can be easily accomplished by using bomb arrows. When killed in this fashion he drops a small key. If the key is not picked up, the game will not recognize that you beat him and he will respawn after you re-enter the room.
The boss of Catfish's Maw is Slime Eel. The fight starts with a tail spawning in the middle of the room and four holes in each corner of the room. With a well timed hookshot you can pull Slime Eel out of the hole when the head pops out, exposing his heart. Only sword slashes damage Slime Eel, and you need to hit Slime Eel a total of 8 times.