Well, this was my initial thought process:
Say I were hiking along the top of a mountain ridge, then just bombed down the open face of it on my snowboard and caught an edge and started tumbling down. Maybe I'll break both my legs, an arm, and get a concussion (for example). What now? I lay there in pain with the possibility of hypothermia. I know I'll have a few hours to live or find rescue. If someone found me, they can rig me up somehow and bring me to safety, and it might just be a painful ride... but I'll live.
Diving scenario: I black out for whatever reason at 100' depth. I can now drown in about a minute. A buddy sees me and decides to bring me up. If there's any air in my lungs, I'll probably have a lung over-expansion, which is deadly - and we'll both get bent - also severly threatening (possible lifelong paralysis) or deadly.
That's where I got my reasoning for things getting very bad very quickly while underwater. A car accident would be more like the snowboarding accident (unless it's an instant death, but I'm guessing instant death in car accidents is a relatively small percentage). You'll still most likely have quite a while to live, whereas in diving, you only have minutes... and you're already in the most foreign and harshest environments for humans.