When David is pulled from the pool and life support is started, Jenny, his wife and ER doctor, places the defibrillation pads on the outside of his wet shirt. In reality, the pads need to be on bare, dry skin so the electrical current created has the ability to pass though the heart impeded. Being on top of his shirt would not allow any contact plus with a very wet shirt on, he would be electrocuted throughout the whole body.
When Jenny Cooper's husband, David, falls on to the pool edge, you can clearly see that his head does not even come close to hitting that edge. Yet when he is pulled from the water, his forehead is bleeding.