Rise of the Tomb Raider: Find a way out of the archives, walkthrough Rise of the Tomb Raider guide, walkthrough

You need to dive under the Greek fire floating on the surface of the water. Use B button, thanks to which you will swim a bit faster. Also, you can try and get some air in patches of water free from fire.

Soon, you will get an rebreather. From now, you can stay underwater for as long as you want to.