What a strange place. I wonder what it is? Maybe you can figure it out. Go inside.
Move the Triangles
Ignore the black and white frozen moblins and darknuts for now, and go down to where there are three triangular blocks and a Triforce symbol. Your boat will talk to you with A. Pull and push the blocks around. They move differently from square blocks. Try not to get them stuck on the raised grey triangles, and get a triangular block onto each of the Triforce triangles. When you've done it correctly, the hero statue will move to reveal stairs. Go down.
Get the Sword
Go down the stairs and go to the little round part of the room. Walk up to the sword to get it. Watch the scene.
Kill all of the Monsters
Now, you have to go back upstairs and kill ALL of the moblins and darknuts. Every last one. But at least you have the Master Sword, right? I guess.
If you can, lure the enemies away from each other so you can fight them separately. The boomerang can help. Don't feel embarrassed if you have to run away if you are outnumbered.
These enemies often drop glowing spheres. Be sure to collect all of the spoils. This is a rare opportunity to gather a large number of spoils at once.
Go Back Above the Water
After you've beaten all the monsters, the doors will no longer be electrified. There's nothing but a semi-invisible wall out back, so go back to your boat and cruise into the glowing spot.