How can I see in the cave

Hint 1 of 5:

Did you bring any glowing gems with you?

Hint 2 of 5:

If not, maybe the swamp is a good place for some deep thinking.

Hint 3 of 5:

Look for a deep spot in the swamp water.

Hint 4 of 5:

Take a deep breath, and dive.

Hint 5 of 5:

Search till you get the gem, and you know the rest already.