This is an excerpt from Scuba Diving Safety by Dan Orr & Eric Douglas.
You can provide open-water rescue breathing in two ways: mouth-to-mouth or mouth-to-mask. Mouth-to-mask refers to rescue breathing using a resuscitation mask or similar barrier device. In today's society, with the increased concern about the transmission of various diseases, giving rescue breaths with some sort of barrier device is highly recommended. Obviously, mouth-to-mouth rescue breathing carries the risk of contact with another diver's bodily fluids, not to mention having to deal with vomitus, which occurs regularly in a diving or submersion incident scenario. Mouth-to-mask rescue breathing may also make it easier to get a good seal and control a diver's airway, especially in open water. The choice of which type of rescue breathing to attempt should be made before you even get in the water. If you plan to use a mask, you will need to make sure you have one with you and that it is prepared for use.
Having a barrier device such as a resuscitation mask available in a diving rescue scenario may not always be an option, however. Even if you keep one in your BCD pocket, it has to be accessible; and unless you've been meticulous about caring for the mask, it may not be usable when the time comes. One problem with keeping a resuscitation mask in your BCD pocket is the fact that it has a positively buoyant, inflated cuff. Divers have been known to reach into their pockets and find that their masks have floated away.
To perform either version of open-water rescue breathing, you need thorough training and frequent practice. In general, the techniques for delivering rescue breaths are the same for both, so we will describe mouth-to-mask rescue breathing. Where appropriate, we will highlight the differences between the two techniques. To begin, with your thumb and forefingers in the shape of a C, seal the resuscitation mask to the diver's face, covering her mouth and nose (see figure 8.3). You will need to press down firmly to completely seal the resuscitation mask to the diver's face.
When you have the resuscitation mask sealed, give two normal rescue breaths, each about one second in duration. Let the first breath completely escape from the diver's lungs before delivering the second breath. Leave your dive mask in place unless you can't get a good seal or deliver effective breaths with your mask on. It will protect your eyes and nose from water and spray. If you have corrective lenses built into your mask, you will need your mask to see properly. It's always a good idea to keep your own mask with you during a rescue. There have been many cases where rescuers jettisoned their own masks during a rescue attempt only to be compromised later when the injured diver slipped out of their grasp and was lost, or when they entered the surf zone and had to try to manage a diver with water in their faces.
In the case of mouth-to-mouth breathing, position your mouth over the top of the diver's mouth, while pinching the diver's nose closed with your right hand. If the diver is much smaller than you are, it might be easier to cover the diver's mouth and nose with your mouth. If you meet resistance when you attempt to deliver the breaths, reposition the head and airway and try again. You may not have gotten the airway open all the way the first time. If you still can't get your breaths in, check the airway for any blockage, such as vomitus or objects such as regulator mouthpiece bite tabs, then clear the mouth and try again.
If the diver doesn't cough and begin breathing again after your two initial breaths, continue to deliver one breath every five seconds. Fill the diver's lungs with each breath. Between breaths, maintain a positive seal on the diver's face and continually reassess for signs of choking and vomiting. As the rescue progresses, check to see if the diver's equipment is impeding your ability to perform rescue breathing. If the BCD appears tight, you may need to release the BCD chest strap or loosen the cummerbund to allow the chest to fully or adequately expand. However, be cautious about removing the BCD and scuba unit. If the diver isn't wearing a buoyant exposure suit, removal of her BCD may make it difficult to maintain surface orientation and provide appropriate emergency care; and if you lose contact with the diver, she may begin to sink. Don't overinflate either your BCD or the diver's. Too much air will make it more difficult to get close to the diver and to get a good mouth seal. If either BCD causes problems with ventilations, it can be partially deflated or, if necessary, removed.
In the water, there is no need to worry about checking the diver's pulse or signs of circulation. Your hands may be cold, the diver's skin may be cold, and wetsuits and drysuits make it very difficult to make a definite determination as to whether the diver's heart is beating. Also, practically speaking, there is nothing you can do about it until you reach a stable platform such as the boat or the shore. The best thing you can do for a diver in cardiac arrest is to get her out of the water and to a place where rescuers can perform CPR and defibrillate her heart using an automated external defibrillator (AED).
This is an excerpt from Scuba Diving Safety.