Judging by the quest's animation, 20 Leagues Under the Sea would have involved the Sole Survivor going underwater and fighting off mutated sea creatures, presumably with a harpoon gun (discussed in further detail below). If for any reason the player was kicked out of the current faction related to the quest, it would default to the Minutemen. A male voice type is attached to this character in the main files, but also completely unused. The quest would proceed once all the synths were eliminated.
This quest would trigger if the player was no longer allied with the Institute, and involved Synths taking control of Diamond City's Radio Tower.Several script remnants still exist for this quest, and indicate the following: The mission would have taken place before The Nuclear Option.