If that’s the case your mouth will fall open. The first thing you learn when freediving using a snorkel is to keep a pressure of air in the mouth piece that’s what keeps the water out.When you pass out, your muscles would relax. I experienced it myself. When I passed out, I wet myself as my bladder was full at the time. Luckily I didn’t injure my head when I fell unconscious. I can imagine your air passage would also relax and open when you pass out.
Glad to hear that you haven’t experienced a SWBO. Make sure you don’t freedive alone too!