In the case of hypoxia on CCR, the victim just falls asleep and survivors of such events sometimes describe recalling a sense of wellbeing or euphoria prior to LOC. There are plenty of cases of rebreather fatalities due to hypoxia with the body resting on the bottom found with the mouthpiece (called a DSV or BOV) still in the mouth and no water in the lungs. Not an exceptional circumstance and perhaps even most likely, although hypoxia on open-circuit is much less familiar to me than closed circuit. However, I'm not aware of any hyperoxia CCR fatality where the mouthpiece remained in mouth, it will always be spit out at the onset of the event.
Over 40 years ago I was at a dive shop as the owner opened, his very young daughter (perhaps 5 or 6 years old) was tagging along. She made a beeline to the back, emerging with her pony bottle. I watched as she climbed up and balanced on the rim of the water bath at the fill panel and proceeded to fill her own tank. Her only challenge was she was not strong enough to open the line valve, but she persisted with both hands until she got it cracked open. She then closed the valve, bled the line and disconnected the whip, jumped down and mounted the pony on the back of some kind of tiny ski-vest looking jacket (apparently cutdown just for her) and attached a standard rental regulator with hoses almost as long as she was tall. She did all this without any questions or assistance, then shot out the side door to the adjacent training pool. I looked at the owner who clearly wasn't paying much attention at all to this and he only said "been doing it since she started kindergarten". I saw her paddling around on the surface (no way was she going to sink wearing was was essentially a life preserver vest) with no fins or mask and happily blowing bubbles with the octo. It was a different era before "helicopter parenting" became a concept, but instructor SOP is students must manage and assemble their equipment themselves without assistance.