I can see how this would be potentially fatal. OK you've got a vehicle with exhaust bubbling in the water. The water is acting like a seal on the floorpan. The gasses collect, and make their way into the cabin through drain holes and seams in the floor structure and through the firewall. Bad news. Heavier than air as I recall, so it would displace all good air and you'd suffocate.

Thanks for sharing that one djkymar. Good off-road safety tip!!