Swim towards the stairs.
You can go right through the fire. You will not be damaged, because you have no health bar. You will sound damaged tho.
Enter the Lighthouse.
You can still move even tho the lights are off (of course).
Descend towards the bathysphere and board it.
It's slightly faster taking the right (counterclockwise) stairs down to the bathysphere, because you don't have to clear the door.
Watch the video, see Rapture go by, listen to Johnny and Atlas talk. These are all ambient audio, not radio messages, so you can't skip them.
This is technically a different level than "Welcome to Rapture" - different saved game names, different load screen (though the load screen calls it "BioShock" where the saved game calls it "The Crash Site").
Not much to do here.