One day Cousteau's prediction of Homo Aquaticus may come true. Men will be engineered to breathe underwater through gills. Until that time we will have to depend upon his other invention: the aqualung. That we must take our own air supply underwater with us just to survive should remind us all that our lives are always in the balance.

Divers depend on their equipment for their safety. Unequivocally. Non negotiable. An absolute. So it's intriguing to watch how divers treat equipment selection, train to use it and care for it.

My friend and diving buddy James Davidson used to parachute. The more he jumped the more he thought about his 'chute failing. On the balance of probabilities the greater the number of jumps he made, the sooner the day would arrive when his luck ran out. A similar rule applies to diving. Think for a moment about how many items of equipment you depend upon on any given dive. Now consider how many corresponding equipment failures could occur.

Here's a brief checklist of the equipment you depend upon on every dive that can go horribly wrong.

Your personal air supply can fail or fizzle out because your equipment stopped working or you can run out because you didn't watch your pressure gauge. Misjudging your equipment's suitability for a particular dive, like diving in cold water without an anti-freezed regulator can result in a lock up that cuts off your air supply or a freeflow that ditches it. Failing to properly estimate your air consumption, or your buddy's if you have to share, will run your tank dry. Countering this really means looking critically at your scuba system and looking at what could fail. That includes building-in adequate gas margins for sharing air with your buddy who may be a much heavier breather than you are. It also means giving careful consideration to carrying an independent alternate air source to eliminate your dependence on another diver in an out of air emergency. A failure of your buoyancy system---BCD, drysuit and weights---can result in either an uncontrolled descent or ascent. Both are potentially dangerous. A fast descent as a result of losing air from your drysuit or BCD can cause you to fall into deep water. Hazards include debilitating narcosis, entering into unintended decompression, exceeding your air reserves, beating the lung (demanding more air than your regulator can supply), oxygen seizures, blackout and squeezes. Counter measures include weighting as close to neutral as possible (the more overweight you are the faster you'll fall), wearing a BCD with your drysuit for back up inflation and running a direct feed from separate air sources to each if possible, using mini cylinders or C0₂ cartridges to provide back up inflation for your BCD, although this remains controversial, or using dual bladder BCDs with independent inflation to each. Divers should also give serious thought to dropping all or part of their weight system to achieve positive buoyancy---drowning always kills, bends rarely does.

Accidental over inflation of a drysuit or BCD or loss of the weight system can result in a fast uncontrolled ascent. Hazards include lung expansion injuries and decompression sickness. A valve failure can lead to a freeflow of air into your drysuit or BCD. Usually the flow rate is quite slow and most hoses can be quickly detached underwater with practice - so practice. If you can't disconnect you'll need to start dumping fast. Again practice reaching for your dump valves, especially the bum dump if you have one because you may want to fin down to slow the ascent and need to dump while inverted. With a drysuit you may have to break a wrist seal or neck seal to dump air quickly. Often drysuits are worn with gloves and hoods that make access to seals difficult. Practice in a pool.

Weight systems are made for easy ditching. The downside is they can be easily tripped accidentally. If you lose your weight system your ascent is likely to be very fast. Breathe out hard and keep breathing out. Retain your regulator so you can breathe in if you've actually exhaled too much or successfully manage to slow your ascent and need to inhale, as well as to stop water entering your mouth. Dump any air in your drysuit or BCD if you can. Flare your body to create maximum drag and slow you down. On the surface try to get buoyant and into a comfortable position that keeps your face clear of the water. If you have been injured the effects may take time to appear. Ideally have your buddy or the boat recover you and consider calling DDRC or DAN for advice even if you feel fine.

Some divers use a double buckle on their weight belt to reduce the risk of accidental release.

It is imperative that you can monitor your decompression status from dive to dive. There's no such thing as a no decompression dive. Even after a dive without stops we continue to decompress for many hours after we surface. To track our decompression status most of us rely on computers. However computers can fail. If a computer fails underwater the immediate problem is how to surface. If you have been closely monitoring your computer and were well outside the stage decompression limits last time you looked then you are probably still within the no decompression zone. Ascend immediately using your smallest bubbles as a guide if you have no other back up instruments. If possible have your buddy escort you so that you can make a safety stop at 5 or 6 metres for a few minutes. Without another set of instruments it will be difficult to estimate and maintain the correct level for a safety stop.

If you are already into decompression when your machine fails you have a real problem. It will be almost impossible to accurately estimate the correct level and duration of the stops you need to complete. At best if you have back up instruments or can surface with your buddy you may be able to make stops at progressively shallower levels with the shallowest stop being the longest.

Once on the surface you will need to stay out of the water for 24 hours or more while your body totally desaturates before you can transfer to tables or an undived computer.

The best way to avoid such problems is to use two computers that are based on the same algorithm. If one fails the other will display the same information. This will allow you to surface normally including making a safety stop or completing any required decompression stops. As a minimum you should carry a timing and depth device as a back up that will enable you to control your ascent rate and properly time and monitor the depth of any stops. Decompression diving on a single computer should be planned in conjunction with tables so that there is a written down default plan you can take with you and follow in case of a computer crash.

Overheating isn't a common problem in diving. Getting cold is. Especially when diving in new temperature ranges or planning longer or deeper dives than usual give exposure protection some thought. Cold will make you uncomfortable and badly affect your ability to concentrate making you open to dangerous misjudgments. Your motor skills will also be impaired making it more difficult to handle self-rescue or to assist a buddy. Decompression dives often involve long chilly hangs where surfacing isn't an option. Make a simulated dive first to see if your exposure suit will keep you warm enough. If not you can still get out. Choose your exposure suit and accessories carefully. For many divers a single suit cannot adequately cover all the conditions they encounter. Choosing your life support system, for that is exactly what diving equipment is, requires great care. Equipment is interdependant and a poor choice of one item may adversely affect another. A BCD with adequate lift for a single tank worn with a lycra skin may be hopelessly underpowered when used with a thick neoprene drysuit and doubles. The regulator that supplies more than enough air on a 12 m current-free dive in the tropics may breathe very differently when confronted with a for-real sharing emergency at 45 m.

As important as choosing equipment is learning to use it. Practice makes perfect. Familiarity with purges, inflators, dumps, releases, etc will all go to making routine dives more comfortable and save precious seconds in an escalating emergency. Equipment familiarity should extend to your buddy's kit and support equipment like radios, telephones, flares, first aid and oxygen outfits.

Don't skimp on maintenance. It is important. Diving equipment does become less reliable with use. O-rings fail. Springs slacken. Gauges become inaccurate. Have it serviced annually by a competent technician through a reputable store or specialty service company.

When we dive we are not Homo Aquaticus. We're more Cyborg. The proper combination of brain and equipment will see us safely back to the surface. Every time.