Diver cover isnt as straight forward as a rigid set of rules would allow, for instance how many Yachties issue an imediate mayday if some falls off? From the divers leaving the surface they are "missing" Some Surface marker bouys are absolute rubbish in all but the calmest of conditions. The longer tube type Delayed Surface Marker bouys are far better for Helm and crew to spot, but suffer from more windage hence compounding the problem of drift. Divers are trained in the eventuality of losing contact with surface cover, though its still unsettling when it happens speaking from experience.
I lost contact with a pair of divers for 15 minutes once as the other pair where some distance away when I recovered them after doing this I searched downtide and locted their marker bouy, although concerned I wasnt panicking as they were still within their theoretical plan. Had I not located them as I did then I feel the correct course of action would have been a Pan-Pan to all stations downtide of my position to lookout.
In the instance of divers drifting out of view a Mayday may not be the correct course of action; IMO the divers are well equiped (or should be) to survive for some time on the surface and (should) have surface location aids to deploy. So they do not unless indicating distress fall into the "grave and iminent danger catergory" - In all events the Coastguard will if they feel it right upgrade the Pan-Pan to a Mayday.
New boat is here, very happy!