To kill the lovebirds using emergency flooding in Ending It in Fristad Rock in Deathloop, sneak to the top level of the pumping station and go into the control room. Pull the lever.
Go back out on the walkway and go around into the adjacent room with red lighting. Pull out the hackamajig and hack the antenna above the flood barrier and leave the pumping station.
Climb up the elevator shaft until you are at the entrance of the pumping station and wait for Fia and Charlie to drown.