Ellie eventually reaches the Aquarium, where she hopes to meet Abby. This spoiler free chapter of the TLoU2 guide describes the infiltration stage that takes place at the end of Day 3.
The Search For Abby
At some point, the boat will stop moving. You are taken by a wave - swim towards land. Head towards the Ferris wheel. Go to the ship, crawl under the net, and climb up.
Go through a hole in the glass and jump down. Continue your way through a hole in the wall and climb a ladder to the top.
Jump to the other side - after a while, you will reach a room with a locked door. You have to go into the ventilation shaft and move through it. Ellie falls downstairs - a dog will attack the girl. Press the Square button to kill it.
Move forward. You will come across a well-known couple from the Jackson massacre. When the fight starts, press the Square button to win.
You have to complete a short sequence in the museum. As Ellie, go to Tommy and Jesse. You will witness a very emotional scene that will end this stage.