Find a way inside the Aqurium
Once you've made it onto land, follow the path ahead and go through the broken fence on the right, now climb up onto the roof and drop down into the building. Go over to the left and slide through the gap in the wall.
Follow the path along and climb up the ladder at the end of the watery section, then drop down and go through the vent in the other room. Eventually you'll fall through the vent and a dog will come to attack, quickly mash [square] to take it down, then go left twice and interact with the bag on the floor to add an entry to your journal.
Finally, go through the door with the crab design on it to view a cutscene. After the scene has come to a close and you're back in the theatre, head to the main room for another cutscene.