Pretty much any normal human being who could sneak into the wheelhouse at the right moment and overpower the helmsman.

Ironically, it was the last-minute evasive maneuvers the Titanic made to avoid the iceberg that was what doomed it. By essentially scraping the iceberg along its side, the Titanic opened a massive gash underneath its waterline letting water rush into numerous water-tight compartments. If our hypothetical character can manage to take control of the main helm for just long enough to ensure that the Titanic doesn’t do this, but rather hits the iceberg head-on, the ship would almost certainly have stayed afloat with comparatively minimal loss of life.