Before you leave this room, Hookshot to the target on the ceiling to find a chest with Stray Fairy #2 inside. Take the stairs and use the key to open the locked door.
Use the Zora Mask and swim to the other side of the bridge and climb up. Use the Deku Mask to bounce over to the floor switch, step on it to generate a chest with Small Key #2 inside.
Grab the key and swim through the northeast chasm past the Dexihand, but not before hitting the Crystal Switch with Double Cutters in this same corner.
In this room you’ll encounter a few enemies. Sink to the bottom and defeat them then hit the Spike Mines with Double Cutters to make them explode; step on the switch to spawn a chest upside-down.
Head to the surface and climb up, then reflect the light into the mirror for a few seconds. This charges the mirror temporarily, so run over to the light it’s reflecting and aim it at the Sun Block.
Defeat the Real Bombchu guarding the chest and open it to acquire the Compass . Head through the locked door to the north.