The diver who is in danger of drowning while on the surface will usually slip quietly under the water, possibly unnoticed by those nearby.
+1...
I'm in Lake Tahoe as a DM some years ago, accompanying two divers on their OW Dive #4 excursion. Nearby is the certified girlfriend of one of the divers, taking a practice dive after a Reactivate session with an Instructor. In poor control from many years of not diving after her certification, she surfaced exhausted from her own dive. I hear her breathing, rather than really see her.
Out of the corner of my eye, I see her push her mask up...and then just disappear. She had neglected to inflate her bcd at the surface, and exhausted, just gave up.
Finning the 15' between us, I grabbed her inflate button a couple of feet below the surface, and up she quickly rose. Her first words were, "I quit."
I saw the couple an hour later trudging forlornly toward the car. One excited new diver; one beaten down former diver - a victim of ?poor preparation, ?poor instruction, ?forgetting the basics?
My only true rescue.