You know us humans cannot refuse to breathe until we die. Once you pass out your brains normal motor skills take over and you start breathing again. I guess that is different for mammals that have to surface in order to breathe.
Although it does make me wonder what part of the process is different. Is it just not being able to get to the surface in time? The lungs full of water from the motor reflex? Or is breathing a concious decision dolphins have to make?