There are many areas in Hyrule where it seems like a longer Hookshot would come in handy. Even within the Water Temple there are switches which Link can't reach with his regular Hookshot. To obtain the Water Medallion and thaw the imprisoned Zoras, Link must find the legendary Longshot.
The Waterfall
The water level is at its maximum height, so Link can travel beyond the locked door directly beside the floating platform. Before unlocking this door, Link should have two small keys at his disposal.
Flip the Switch
At the top of the waterfall is a room with a switch in the middle that controls the water height. Through experimentation, Link can use his Hookshot to hit the switch and make his way to where a giant Like-Like is lurking.
Mirror Image
Link will find a strangely familiar foe lurking in the shallow water of the next room. This dark figure mirrors Link's every move, which makes it one of the most difficult mini-bosses in his quest.
Key for the Road
Once Dark Link vanishes, the magical spell on the room is broken and the door is unlocked. The Longshot is found beyond the unopened door, and behind the chest is a mystical block bearing the same symbol as the Door of Time.