There is a tendency among divers to inflate their BCDs until their shoulders and much of their chest are out of the water. I see little benefit in doing that. In reasonably calm conditions, I want the waterline around my chin, with my mouth and nose comfortably clear. My arms and legs should be doing absolutely nothing to keep me afloat.
Adding more air beyond that point doesn't necessarily make me safer or more comfortable. Depending on the BCD, it can actually make my position less stable. A heavily inflated jacket may become uncomfortable around the torso, while an overinflated wing can push a diver forward when they're trying to remain upright.
The amount of air I need is determined by how comfortably I float, not by how much of my body I can lift out of the water.
Getting Comfortable at the Surface
After surfacing, I add enough air to establish positive buoyancy and stop kicking. Once I'm floating comfortably, I can deal with finding my buddy, locating the boat, or deciding whether I need to swim somewhere.
In calm water, the test is simple. Stop moving your fins and hands, relax, and see where you settle. If you have to kick occasionally to keep your mouth above water, add some air. If your shoulders are riding high and the BCD is pushing you into an awkward position, try letting some out.
There's nothing magical about the chin as an exact measurement. Exposure protection, equipment configuration, body position, and surface conditions all affect where the waterline settles. I use it as a practical reference because it generally gives me enough flotation without making the BCD unnecessarily full.
In waves or chop, I may add more air to maintain comfortable positive buoyancy. The waterline will constantly change as waves pass, so I'm more concerned with remaining supported and keeping my airway clear than maintaining a particular position against the water.
How Much Air Do You Need?
How much gas is required to float comfortably depends heavily on weighting. Every unnecessary pound of lead has to be supported by additional buoyancy, and a diver carrying substantially more weight than necessary may need a heavily inflated BCD simply to remain at the surface.
I prefer a balanced rig. I want enough weight to hold a shallow stop near the end of a dive with a nearly empty cylinder, but not so much negative weight that a complete BCD failure makes a controlled ascent impractical. Proper weighting also means the BCD doesn't need to carry large amounts of unnecessary gas throughout the dive.
Different configurations have different requirements. A thick wetsuit, steel cylinder, stainless backplate, or large amount of integrated weight can change how much buoyancy is needed. For a more complete discussion, see Buoyancy Control and Weighting and Integrated Weights vs. Weight Belts.
Once the equipment is properly configured, the remaining question is fairly straightforward: how much air lets you float without working?
Jacket BCDs and Back-Inflation Wings
A jacket-style BCD distributes some of its buoyancy around the sides and front of the torso. Many divers find that it naturally supports an upright or slightly reclined position at the surface.
A back-inflation BCD or wing behaves differently. Most of the buoyancy is behind the diver, and forcing yourself into an upright position can become uncomfortable, especially when the wing is heavily inflated. I've seen divers struggling to sit vertically in equipment that would support them comfortably if they simply changed position.
When I'm wearing my backplate and wing, I usually roll onto my back. The buoyancy is now underneath me, my torso is supported, and I can float comfortably without fighting the equipment.
It's also my preferred position for most surface swimming. I can rest or begin swimming on my back while keeping my face out of the water. In calm conditions, I can breathe normally without using my regulator or a snorkel.
I don't see much reason to force a wing to behave like a jacket. Learn how your particular BCD supports you and use the body position that works best with it.
Inflating, Deflating, and Descending
The power inflator is the normal way to add gas to the BCD. At the surface, I use short additions until I'm comfortably positive, rather than holding the button down until the bladder is full. The BCD should be supporting me without becoming unnecessarily tight or awkward.
Deflating takes a little more attention to body position. Air collects at the highest point inside the bladder, so the exhaust valve you're using needs to be positioned where that air can escape.
With a conventional corrugated inflator hose, raising the hose above the bladder and pressing the oral inflation/deflation button allows air to escape. Depending on the BCD, shoulder or rear dump valves may be easier to use in other positions. A valve located below the trapped gas may release little or nothing until you change your position.
This becomes particularly important when preparing to descend. I want my buddy and me ready to descend together before I begin venting the positive buoyancy that was keeping me comfortable at the surface.
With appropriate weighting, releasing that gas and exhaling normally should allow a slow, controlled descent. As my exposure protection compresses, I add small amounts of gas to the BCD to compensate. Waiting until you're falling rapidly before trying to restore buoyancy makes the descent unnecessarily difficult.
The same principle applies during an ascent. Gas expands as ambient pressure decreases, so the BCD needs to be vented as necessary to maintain control. For more on the underwater portion of the process, see Controlled Descents.
Rough Water and Equipment Problems
In rough water, I generally keep my regulator in my mouth. Waves can cross your face even when you're comfortably positive, and I would rather have a dependable breathing source than repeatedly deal with water entering my mouth or nose.
I also don't reduce inflation simply because waves are pushing me around. If the BCD is supporting me properly, I can concentrate on positioning, staying with my buddy, and getting back to the boat or shore.
If the power inflator stops supplying gas, the BCD can still be inflated orally as long as the bladder and oral inflation mechanism are functioning. Bring the mouthpiece to your lips, press the oral inflation button while blowing into it, and release the button between breaths to retain the gas you've added. Repeat until you have enough buoyancy to float comfortably.
Oral inflation is worth practicing occasionally with your own equipment. It isn't complicated, but knowing exactly how your inflator works is considerably better than trying to remember a skill you last performed during certification.
A power inflator stuck in the open position presents a different problem. If gas continues entering the bladder after you release the button, disconnect the low-pressure inflator hose and vent the excess gas. Underwater, this needs prompt attention because continued inflation can produce an uncontrolled ascent. The malfunction ends the dive, and the equipment needs to be corrected before it's used again.
At the surface, the immediate priority is establishing and maintaining positive buoyancy. If the normal inflation method is unavailable, use oral inflation or get assistance rather than exhausting yourself trying to remain afloat.
Helping Another Diver at the Surface
The same principles apply when assisting a tired diver. If someone is exhausted and still kicking or sculling to keep their airway above water, they haven't established adequate buoyancy.
My first concern is getting them comfortably positive so they can stop working. Depending on the equipment and their condition, that may mean adding BCD gas, helping them recline onto their back, or supporting them while the situation is brought under control.
Once they're floating comfortably, it's much easier to communicate, assess their condition, and decide whether they need a tired diver tow or assistance from the boat.
I don't routinely plan around ditching weights because I prefer a balanced configuration that doesn't depend on it. However, not every diver has that equipment arrangement, and not every situation can be resolved by adding air. Releasing ditchable weight can be an appropriate emergency response when adequate buoyancy cannot otherwise be established, particularly during a rescue.
That's one reason I want to know how my buddy's weight system works. Integrated weight pockets, belts, and other configurations have different release mechanisms, and an emergency is a poor time to discover that you don't know where someone's weights are carried.
For ordinary surface buoyancy, the standard remains the same whether I'm helping someone else or managing my own equipment. I want enough lift and the right body position that the diver can stop working and float comfortably, without using more inflation than the situation requires.