So basically combining shallow water blackout with plastic bag asphyxiation. It would require some timing to place the plastic bag before passing out.
A few years ago I was doing the Wim Hof breathing method (has nothing to do with ctb-ing, but for health) which is basically hyperventilating and then holding your breath. I felt myself being dizzy but passing out would be hard. But interestingly, several people died doing the Wim Hof method, due to passing out while in the water or driving a car.