Thought you might be interested....
When I trained to be a ship's radio officer we were taught that all distress R/T traffic should be prefixed and terminated by the word MAYDAY. eg "MAYDAY XXXX this is YYYY twenty souls on board MAYDAY".
The rationale was that it would quickly be obvious to stations switching to the frequency in use that distress traffic was in progress and that they should stay off unless they had something useful to say. It was considered that this far outweighed the time spent saying MAYDAY twice.
It has to be said though that emergencies at sea tend to progress more "slowly" than those in the air so cluttering up R/T traffic with MAYDAY may not always be appropriate.
As with all things, apply a little common sense and you won't go far wrong.