thought her air ran out...the water level kept rising up eventually I think but I thought I heard the cause of death was actually suffocation. Who knows, maybe was drowning but the D.A. (acc. to Wikipedia) left some doubt:
>>citing blood found on Kopechne’s skirt and in her mouth and nose “which may or may not be consistent with death by drowning”.
An air pocket would remain in the car even if she breathed whatever usable oxygen was there before it ran out. The oxygen (air) would be replaced with the CO2 she exhaled.