The Queen Leviathan is in a long cave off the northwest corner of Skeleton Reef a skeleton will point the way. You'll have to pull down a statue using the magma bombs as holdfasts to get to the end of the cave. You need to feed her three things that look like thistle blossoms. A yellow one is nearby just below her. The other two, pink and blue ones, are on the other side of the statue. It's a little tricky to pull down the statue, then switch the claw to grab the blossom and stream back past the statue before it comes back up and blocks the way again. Remember to swim towards the beast and release the claw to launch the food into her mouth. If you feed her the wrong things she will spit them out. When you feed her the right things her eyes will twinkle.