While 20 years ago a diver’s biggest fear was to be
attacked by a great white shark, since the film Open Water
the biggest fear is to be adrift at sea. Of course it’s more
likely that a diver will get lost at sea than attacked by a
shark, given the number of cases seen.
Usually, a stranded diver is recovered within a couple
  of hours, either by a rescue team or their dive boat. Still,
  divers can face a lot in two hours: hypothermia, dehydration,
  high swells, severe weather conditions and panic,
  for starters. And it doesn’t matter how warm the air temperature
  is. If you get separated from your dive boat, you
  can increase your chances of surviving by following some
  important procedures.  
Stay Together
 If you surface away from the boat, stay with the other
  divers. Be cautious in heavy swells when there is a risk of
  injury from bouncing off one another’s equipment. Keep
  your equipment on; divers who have jettisoned gear later
  say they would not do it again. Their position in the water
  is more comfortable with the tank, inflated BCD and
  weight belt in place.  
Protect Yourself from the Elements  
Survival experts say that a diver should protect himself
  first before looking for a way out of the situation. The biggest danger divers face is the cold. Dr. Glen Egstrom,
former NAUI board member and UCLA professor says,
“Recognize that you are in peril and what you are wearing
constitutes a form of shelter.”
 Mike Bothma, an experienced dive guide at Camel
  Divers in Sharm el-Sheikh, Egypt, says “Hypothermia is
  a problem even in waters of 75ºF.” Body heat loss is 25
  times greater in the water than in the air. Avoid motions
  that will increase your blood circulation, because they
  increase your body heat loss. If you can, get into a fetal
  position: raise your knees, cross your legs, tuck in your
  elbows, and cover your face with your hands. Huddle with
  a buddy to keep warm. Face each other and cross arms
  — this encourages face-to-face contact, which is good for
  morale and improves your chances of being spotted by
  the searchers. If someone is too cold or tired, he can be
  placed in the middle of this huddle to warm up. Figure
  out how you might tether yourself to your buddy to stay
  together.  
Sunstroke is a danger. Try to shade your head
  with a piece of clothing – this will also help to
  protect your eyes from the bright reflection of sun
  on the water. Don’t drink salt water, because it will
  increase your rate of dehydration. Do not retain
  urine. Keep calm. Focus on the positive. Remember
  that the boat knows you are missing and someone will be
  looking for you. If one of your fellow drifters is panicky,
  reassure him. Egstrom says that morale can be boosted by
  telling jokes, praying, enjoying favorite memories or fantasies,
  and venting anger.  
Location  
Don’t swim for shore unless you are sure you can make
  it. A woman diver in Belize died last year because she
  couldn’t make it to shore. There are many devices on the
  market for attracting attention in an emergency. These
  range from a small CD used to reflect light to an expensive,
  electronic EPIRB radio beacon. Maintain maximum
  buoyancy to float high. Use whatever means possible
  to display your position. If the water is shallow, attach
  weights to any line you might have and drop it to the bottom
  to act as an anchor, to avoid drifting further.
 Don’t Get Lost in the First Place 
Dramatic changes in currents or a boat breaking down
  or becoming untied are main reasons a diver gets separated
  from his boat. Add to that diver error – not remembering
  how to get back to the boat, for example, running
  out of air too distant from the boat, or failing to carry
  any signaling device. To prevent yourself from getting left
  behind, there a number of steps to consider:  
Ask another diver on board to ensure that you are on
  the boat before it departs. Offer to do the same for him.  
If you and your buddy plan to stray from a leader during the dive, let the leader and captain know. Create a
  directional plan, including a time you expect to be back
  under the boat and be sure to let them know.  
Don’t dive if you aren’t comfortable with the conditions.
 And, carry rescue aides  
Signaling flag: This folds up and is usually attached
  to your tank, where it can be pulled out if needed.
  Researchers at England’s Heriot-Watt University studied
  the visibility of various signaling devices and found that
  folding flags were by far the most reliable and cost-effective
  location device tested. Yellow was the most conspicuous
  color in all sea states, even with breaking waves and
  deteriorating light. A day-glo yellow pennant was consistently
  spotted at more than 1.2 miles to 1.8 miles. Red
  and orange flags were only visible to a mile away. If your dive shop or boating supply store can’t order one for you,
  you might consider making your own.
  
    | It’s a mystery why many divers continue to purchase black or otherwise dull-colored wet
 suits and BCDs that can’t be spotted easily.
 | 
Surface marker buoys (SMBs): SMBs or delayed
SMBs with reels are the most common signaling devices
when diving. Standard SMBs are permanently inflated
at the surface throughout the dive. Delayed SMBs such
as brightly colored safety sausages can be inflated and
deployed underwater. The advantage of standard SMBs
is that the boat knows where you are for the entire dive.
However, when diving a wreck, a delayed SMB may be
used to prevent lines being caught. Safety sausages can be
inflated from your tank, sealed shut to stay upright at the
surface, and even illuminated from within. The taller and
wider they are, the more visible from the boat. The
most visible would be same day-glo yellow as the folding
diver’s flag. For more info see www.halcyon.net or
www.omsdive.com.
RescueStreamer: In the June 2001 Undercurrent, we
  reported on Navy tests of the RescueStreamer, a bright
  orange device visible to the naked eye from an altitude
  of 5000 feet when fully extended on the water’s surface.
  To improvise, if you hear a search plane overhead,
  you may be better off deflating your safety sausage
  and stretching it out on the surface so it can be spotted.
  (www.rescuestreamer.com)  
Strobe and Light: Powerful lights and strobes are
  vital when it is dark. Lights that produce very bright
  beams are visible 2.5 miles in daylight and 5.4 miles in
  darkness. They’re most visible when moved slowly and
  steadily in a horizontal and vertical scan rather than pointed at the search vessel. Carry a spare at night, because
the primary light may have lost power – the Sunlight D8 from
Underwater Kinetics (www.uwkinetics.com) has a long burn
time. A high-intensity strobe is useful in low light, but it needs
to be mounted as high as possible, such as on the top of a folding
flag. The submersible Mark-Lite strobe will operate for six
hours on one AA alkaline battery or 10 hours on a lithium battery
(www.sunbrite.com/sun07.htm). A camera strobe is also a
helpful signaling device.
Whistle: Several types are marketed by Trident Accessories
  (www.tridentdive.com). While whistles attached to BCDs are
  lightweight, cheap and small and will not run out of a power
  source, their range is extremely limited and may not be heard
  over the boat engine or wind. Don’t count on them.  
Air horn: Good for location in fog or at night. Models
  such as the Dive Alert or Aquatec Sub Alert attach to the BCD
  inflator hose (www.divealert.com or www.aquatecusa). They’re
  substantially louder than a whistle, but also lose effectiveness in
  winds and are useless in an air search.  
Waterproof flare: Small flares in waterproof canisters often
  consist of a red candle flare at one end and a smoke flare at the
  other. PowerFlare (www.powerflare.com) is battery operated
  and submersible to 300 fsw.  
Reflective disc: Old CDs or compact mirrors make excellent
  reflective devices. In a pinch, a dive mask or shiny knife blade
  might suffice.  
EPIRB: Emergency position indicating radio beacons
  (EPIRBs) are small electronic devices that transmit distress
  signals to aircraft and ships. They are becoming standard issue
  at remote dive sites, but the best ones are expensive. A few can
  be taken underwater, such as the Sea Marshall PLB8-LD Diver’s
  Beacon (www.seamarshall-us.com), and the Guardian MOB
  watch with an integrated honing signal (www.mcmurdo.co.uk).  
Brightly colored gear: It’s a mystery why many divers continue
  to purchase black or dull colored wet suits and BCDs
  that can’t be spotted easily. Buy yellow or orange gear, wear
  a brightly colored hood. Wave colored fins or mesh bags to
  attract attention.  
PS: Many liveaboards offer rescue equipment for passengers.
  Bilikiki Cruises (Solomon Islands) gives out free safety
  sausages. Peter Hughes’ Sky Dancer in the Galapagos requires
  each diver to carry a safety sausage, Dive Alert air horn, plus
  mini-strobe light on all dives, and highly recommends an
  EPIRB. If you don’t bring your own, they can be provided,
  but quantities are limited.  
Some boats employ a diver recall system, generally an
  audio alarm that calls all divers back to the boat in an emergency.
  One Ocean Technology Systems model can transmit
  voice instructions over a 100-yard range, which can be heard without a special listening device (like a pager in a
  supermarket). Or it can send out a longer-range tone.  
Much of this article is based on a piece that appeared in the British magazine Diver, as well as articles previously published in  Undercurrent and stories of divers who have drifted at sea.