In one of the “The Making of...” features Cecil B. DeMille was asked about the parting of the sea. One night, exhausted and depressed about the special effects department being stymied, he had the sudden inspiration of filming the water with the cameras upside down and then splicing the effect into a rightside up view.
This says they they dumped the water in, then ran the film in reverse.
https://www.youtube.com/watch?v=4VHS3P6HW6Y