 The scenes of crewmembers swimming outside the sub were shot on dry  soundstages with the actors suspended from wires. There was some  additional hazard involved because, to avoid reflections from the  metal, the wires were washed in acid to roughen them, which made  them more likely to break. To create the impression of swimming in a  resisting medium, the scenes were shot at 50% greater speed than  normal, then played back at normal speed.