I was talking to my flight nurse 'missus' about this last night and they only ever need to worry about transporting divers from lakes which are higher than the closest hyperbaric chamber. She said they would keep as low as possible, but obviously not too low that they hit wires and kill four people. If she thought she had a problem she would patch into the docs at the chamber. She did say that a helicopter transport would only take around 30 minutes, where if you sent them by ground it would take several hours. So in an emergency situation I guess you also want to consider the risks/rewards of leaving them at a lower altitude but for a far longer period of time.