Mike and Tony Petrakis are a Greek father and son team who dive for sponges off the coast of Florida. After they are robbed by crooks, Arnold and the Rhys brothers, Mike decides to take his men to the dangerous 12-mile reef to dive for more sponges. Mike suffers a fatal accident when he falls from the reef leaving Tony to carry on the business. But now he has a companion, Gwyneth Rhys.
| Actor | Role |
|---|---|
 Robert WagnerTony Petrakis | Tony Petrakis |
 Terry MooreGwyneth Rhys | Gwyneth Rhys |
 Gilbert RolandMike Petrakis | Mike Petrakis |
 J. Carrol NaishSocrates Houlis | Socrates Houlis |
 Richard BooneThomas Rhys | Thomas Rhys |
 Angela ClarkeMama Petrakis | Mama Petrakis |
 Peter GravesArnold Dix | Arnold Dix |
 Jay NovelloSinan | Sinan |
 Jacques AubuchonDemetrios Sofotes | Demetrios Sofotes |
 Gloria GordonPenny Petrakis | Penny Petrakis |
 Harry Carey Jr.Griff Rhys | Griff Rhys |
 Rock HudsonNarrator (uncredited) | Narrator (uncredited) |
| JJack EnglishDoctor (uncredited) | Doctor (uncredited) |
 William JohnstoneCrewman of 'Snapper' (uncredited) | Crewman of 'Snapper' (uncredited) |