This chapter of The Last of Us 2 guide contains a spoiler free walkthrough for The Forward Base/The Aquarium level.
The Forward Base
Follow Manny inside. If you want, you can talk to Abby's friends on the way. Go to the hospital, where you meet Mel - after the conversation, follow Manny and Nora. Go over to the body and identify it, and after talking, enter Isaac's quarters.
After talking to the leader of the wolves, a flashback begins. After the cut-scene, follow Owen. You'll get to the aquarium - follow the boy. When you reach the bay, walk up to the boat and board it.
After talking to Owen, enter the Aquarium. Approach the locked door and you'll reach the main room of the facility. You have to look at the other interesting objects found in the room - paintings on the walls etc.
Go up the stairs and pull a note out of the skeleton's hand. Now go downstairs and, with the help of the key, open Max's Place. Go to the elevator and continue with Owen to explore the underground location. After the cut-scene, the stage is over.