In this spoiler free chapter of our game guide to The Last of Us 2 we have prepared a detailed walkthrough of the Return to the Aquarium stage. It's a short stage, which takes place right after the hospital, where Abby talks, among others, with Mel and Yara.
After the cutscene, you have to follow Yara. You are looking for Lev, who has disappeared somewhere. Go through the next rooms of the Aquarium and search for him.
Despite your efforts, you won't find the boy anywhere. At some point Bear will show up. Pick up his mascot and throw it to encourage the dog to fetch it. Wait for Yara to play with the dog. After that, go on.
When you reach the main room of The Aquarium, together with Yara, you will start thinking about a gift for Lev. Approach the shark plushy and pick it up. After the cutscenes, you'll find out that Lev escaped and you have to follow him.