In the late summer of 2001, James Cameron, the director-producer of the highest-grossing picture in Hollywood history, led a new deep-diving expedition to the wreck of the lost liner Titanic. With him was a team of underwater explorers that included the artist Ken Marschall, the historian Don Lynch, and two actors from the movie, Bill Paxton and Lewis Abernathy (who played Brock Lovett and Lewis Bodine). Their equipment included state-of-the-art digital...