You would think that seeing your dive buddies swim to the surface as fast as they possibly could would be all the visual cue you would need. However, a very simple solution that works in any conditions is clickers. I don't know if you have seen them at huge public events. They pass them out at bike races here, criteriums to be exact. It's a simple device. A rigid piece of metal in a thumb sized plastic encasing. When you depress one end with your thumb it creases the medal band and creates a loud popping sound that carries one heck of a long way under water. A series of different clicks could be used to communicate a series of different things. A handy tool and an ancient concept. I think you can get them from on-line party stores for about 50 cents a piece but don't quote me on that.
My name is Inigo Montoya, you killed my process, prepare to die. Slightly modified quote from Princess Bride. Code-frog System Architects, Inc.