Here's a possibility… perhaps they're in the front seats with car running so they can run the heater. She accidentally hits the gas. The car plunges head first into the water. Water fills up the front seats first leaving air in the back seat area. Water is filling up the car. They aren't able to push the doors open due to the pressure. They scramble to the back seat for air. Water finally fills the back last and they hold each other as they drown in the back seat. Horrible. God rest their souls.