Robert Fulton, for the purposes of disambiguation.
Talk about it here.
He developed a prototype submarine for Napoleon called the Nautilus. The Countess enhanced it with an omega drive, but the Second Doctor sabotaged it so it and the submarine would explode, keeping history from being altered. Fulton and the Doctor escaped prior to the destruction. The American could not swim, so the Doctor had to save him from drowning as well. Afterward, the Doctor advised him to give up on submarines and focus his research on developing a viable steamboat. (PROSE: World Game)