you must save the zora boy. his dead mother will then give you a new piece of clothing to wear. it will allow you to breath underwater.
you get it in the temple at the bottom of lake hylia
At the underwater temple, beneath lake Hylia.
I'm pretty sure its at lake hylia, if not, figure it out.
you can get it in lake hylia i think so.
lake hylia
Get the silver scale in Zora's Domain, then go to Lake Hylia through the warp door. The letter is near the door at the Lake Hylia end.
you need to go to the lakebed temple in lake hylia
You need to get the Longshot, the main treasure in the Water Temple in Lake Hylia
Fyer is in Lake Hylia, and owns the cannon business that floats on a platform in the middle of the lake.
You find them in Zora's Domain, Castle town and Lake Hylia
You must beat the water dungeon, then talk to the canon guy in lake hylia.
If you're talking about in twilight princess, there is a lake bed temple. Complete all the steps to the temple. You should eventually end up in an underwater maze. At the end, there is a room with three spider web covered holes in the floor. Cut the one in the middle, fall through, and the boss' key should be beneath you.