There would also be the risk that the person would start to recover underwater and take a breath.
Not a good idea, except maybe in exceptional circumstances.
I once saw something like that in a TV documentary about accidents and first aid. There was a reconstruction of an accident in which a boy got his finger stuck in a drain at the bottom of a swimming pool. The people who rescued him were giving him air from their lungs underwater, because they had not yet managed to free him. Fortunately he survived.
Lucia
