First open your map. If you’re anywhere north of the Boiler Room after defeating DeathStroke, then chances are you may face this problem. You cannot get back to the Boiler Room by going the way you originally came. The door will be locked when you reach the entrance.
The exit is all the way to the south. Getting there could be tricky.
1. Head to the Deck Access Corridor. Here's where it is on the map:
2. Go all the way down to Deck 3. It's the platform at the same level as the water.
|Just look for this view to know you're in the right place. Do not Batclaw into that platform. You should turn around and see what's behind you.|
3. There's a wall that can be destroyed with the Explosive Gel (you may have already destroyed it earlier). Enter the passage. Instead of crawling under the prominent opening in front of you, look all the way up.
|Once you see this vent, look up.|
4. You should see R1 flash on your screen that says you can use the Batclaw. It's a vent that allows you to go through the Deck Access Corridor.
The vent ultimately leads to the Boiler Room. Once in the said room, everything is straightforward. Continue heading south until you get to the exit.