Producer Roger Corman got the idea for the movie while reading a Los Angeles Times article about a one-man submarine manufactured by Aerojet General. He phoned them and asked them if he could use it in a film, telling them that he couldn't pay them but they'd get free publicity. According to Corman they were delighted.
The mini submarine in the movie is a Aerojet General. This is a electric mini sub that was designed for the Navy's demolition teams. Roger Corman was able to use it for free for the movie because the designers wanted the publicity. They had hopes that the public would see it and want one for recreational activities.