Head to Badtown's bar and talk to Buck. After a short conversation, head to the next mission marker and find yourself a boat. Head to the Nostalgia and board it. Clear the enemies outside then open the bulk door to enter it.
Once inside, continue killing enemies until you reach the last room in the lower deck. There's a heavy guarding it so stay back and lob and explosive inside to kill him instantly. Examine the laptop and watch the next scene.
You'll have to escape the sinking ship so follow the straighforward path and press L3 to swim faster. You may want to stop and interact with the oxygen tanks to catch up with your breath. Continue swimming until you escape the ship.