Once you've completed all of your tasks inside of the Water Temple, you will enter a large chamber where Corallis, the water boss will emerge from the water. In this boss fight, you'll have to rescue your Aunt Lenya by defeating the boss itself.

As the battle begins, Corallis will sometimes hover to one end of the "pool". The trolls above on the pillars will more than likely blast their explosive, plant balls toward him. Move over to ramp-like object where Corallis is and if there are any bomb plants in sight, use Major Ruin to carefully hit them off of the ramp, and make them fly onto Corallis. Once Corallis is deep underwater, due to the explosion, jump into the water and transform into Deep Blue.

Now, here's where the battle gets a little tricky. You will QUICKLY have to use Deep Blue underwater to fire at a tentacle until it pops, then QUICKLY swim out of the water pool. Once Corallis rises, he will shoot lazers from his tentacles. Run away from the electric charges until they are gone, which should not take long. Repeat this process until all of the tentacles are gone, and Corallis will be defeated. Once your Aunt Lenya is free, she will award you with your old, warrior companion, Chilla.