1. The vehicle had sunk.
2. It means March was not immersed in the water and his head was above the water level.
3. He was out of breath./He ran out of air.
4. David, a yachtsman, found the van first.
5. The sentence is "The van lay invisible in murky darkness, five metres down.".
6. He tried to open the door but it would not budge.
7. He tied the rope to the handle of the door of the van so that it would be easier for him (or other rescuers) to get back to the van to try to rescue Nancy.
8. There was no time to waste in saving Nancy.
9. The firemen had done a good job. They managed to save Nancy.
10. She survived the accident and was attended to by a nurse.