How to Beat the Giant Moray Eel (Eel Boss)

The fight

1. Don’t fight it in deep water — you can’t damage it while the water is high.
2. Turn off the 4 handwheels around the room to drain the water.
3. The eel ends up lying on the bottom. Attack it now.
4. After roughly 15 seconds it brings the water back. Drain it again and repeat until it dies.

The game files list the eel at 900 HP with 20 contact damage, so expect several cycles. Watch the fight in the playthrough linked below if you get stuck.

Reward

The Guarana Gem, one of the four gems needed for the endings.

The hidden room

For the true ending, a hidden room opens on the right side of the eel boss room once all 17 treasures are returned.