As a Navy Helicopter Pilot I have had training in, and experience conducting Search and Rescue. None of my experiences have been to find overdue divers but they have been to find people lost in the Oceans.
Chance are (hopefully) none of us will be left by a boat or swept out to sea by currents but in the event you find yourself in this predicament here are some things to help you out.
Here are MY OPINIONS (disclaimer) on what TYPE of gear is best to signal an aircraft/rescue boat(i.e. Coast Guard/Navy). This is based on my experience in actual search and rescues. I'll let the experienced divers here fill in what specific gear they like.
Nothing earth shattering here but may serve to let New Divers know what works well.
Everything listed here is good but their effectiveness is more than doubled if the searchers know you have the equipment. Leaving a list of signaling equipment on the boat/ with a responsibile person along with your dive plan will greatly improve you chances of being found.
#1 Signal Mirror.
PROS: Cheap. Never runs out of batteries. Very bright on a sunny day. Can still be seen on overcast days but not as bright. WILL get the attention of a pilot/boat
CONS: Useless at night (nothing to reflect). Have to see the rescue craft to reflect light towards them. (CD's are not as good as a mirror)
#2 SMB (or large bright colored object) Size does matter!
PROS: Cheap. easy to use. Easy to see! Helps pilots to see you in the troughs of waves.
Cons. Bigger the better....means can be bulky.
#3 Sea DYE Marker
PROS: Cheap. Passive marker. If you are tired/incapacitated you don't have to do much to deploy.
CONS: Choppy/strong current/rough waters tend to dissapate quickly. Useless at night
If you dunk it to make a "Dye slick" and then put away until slick goes away and repeat - it'll last longer
#4 Smoke Marker/flare.
PROS: Everyone investigates smoke. Seen from large distances.
CONS: Water friendly ones are expensive. Short burn time. Pyrotechnic.
#5 Light/ Strobe light
PROS: Strobe in the day time is O.K. Excellent at night. Aircraft on Night vision will see more than 30 miles away. These light up the sky on night vision.
CONS: Light good at night but rescuer must be looking directly at in the daytime to notice. Batteries run out.
#6 Glow Stick.
PROS: Last 12 hours. Will be seen by Night vision.
CONS: Harder to see with naked eye at night
#7 Air horn/whistle
PROS: Will get attention without anyone looking at/near you.
CONS: Limited range. 1 mile for Air Horn/much less for whistle. Useless for aircraft.
#8 ELT Emergency Locator Transmitter/Radio
PROS: Will give a signal right to you!
CONS: Cost Prohibitive. Bulky. False alarms from those who are not 100% familiar with their operation.
In my Emergency kit I have a Mirror, Sea Dye Marker, SMB, Whistle (came with BCD) and a strobe. I feel these give me the best spread for any condition and also minimize bulk.
Please feel free to add anything I may have missed and your thoughts on specific gear.