Imagine the scene. You've been under water, engrossed in your dive, for about an hour. Finally making it back to the surface, with your buddy, you are dismayed to find no sign of your cover boat. It happens. The sea is a big and lonely place under these circumstances.
Your boat may be miles away, its crew distracted by engine failure and a prevailing wind, or it may be very close at hand, hidden behind the height of the waves. Either way, at some point they'll be looking for you, and two tiny heads bobbing amid the spume don't make a very visible quarry.
Brightly coloured gear can help. Some drysuits have light reflecting and radar-reflecting patches attached. I've got a white stab-jacket which has proved to be of value when used in the expanses of water in remote areas. But black is now the fashionable colour for diving gear - and that makes a diver almost unnoticeable at the surface.
So imagine you're floating about, dressed in this year's colour. You're feeling exposed and insecure. It's at this point you start thinking about attracting attention. How do you go about it?
Whistles can tell your boat cover that there are divers at the surface. Not the puny little life-jacket whistles that were a permanent feature of ABLJs. I'm talking about the type that connects in-line with the direct feed of your BC and emits an ear-shattering scream that can be heard for miles around. Underwater diver-to-diver alert units can also work at the surface - but are less effective. However, if you've ever had the job of picking up divers, you'll know that even if you can hear a whistle, you can't always tell where it's coming from. If the divers have no visual device, you've got little chance of spotting them.
Bright flags on telescopically extending poles are one idea. Another is the long, day-glow sausage which can be inflated by mouth or regulator to stand loud and proud up to 6ft above the surface of the water. These have the advantage that they can be rolled up and occupy little space in a BC pocket - but beware of seductively cheap versions which have all the longevity of a supermarket carrier bag. There's nothing worse than finding that your sausage has turned into an open-ended tube.
If you're diving in an area with reliable sunshine, a small mirror can be useful for flashing back at the boat. You can also use the reflector of an unlit diving torch. Naturally, at night a torch is an essential signalling device. Most dive boat skippers now assume any white light on the surface means divers need picking up.
Flashing strobes will send their signal for a much longer period (up to 8 hours on one set of batteries), and will allow the pick-up boat to differentiate between divers coolly waiting and a more urgent appeal. Underwater photographers' electronic flashguns often have a lower-powered repeat flash setting for use in this way. Of course, a surface marker buoy allows the divers to be kept track of while under water, if their boat has managed to keep with it.
Late-deployment SMBs, those a dive pair will put up only immediately prior to their ascent, should be large and stand to attention in a sufficiently attention-grabbing way; care should be taken not to allow such a buoy to lie recumbent and invisible. There are late-deployment SMBs now on the market which have a constriction at their filling end to prevent them from losing their air if they happen to fall over at the surface.
At night, it's a good idea to fix a permanently flashing strobe or chemical light stick to your SMB.
What if you've done all these things and still you are left floating without a pick-up boat?
This brings us into the realm of real emergency equipment, such as flares which are designed to be taken deep on a dive and still ignite - either to make smoke or to fire up into the sky, descending on a small parachute.
The latter versions can withstand water pressure up to 5bars (40m), can be hand launched, reach an altitude of more than 350ft, burn for around 8 seconds, and float. Of course, they only work once, and you don't know if they are going to work until you try, so be sure there's someone in sight to see your flare before you set it off. Then there is the diver's personal location beacon (PLB), which is a low-powered emergency position indicating radio beacon. PLBs are excellent for locating divers lost on the surface by anyone equipped with a locating device and knowing to look for them.
Go correctly equipped and you can look cool and in control at the surface, arm raised high, and hand displaying the correct CMAS-approved signal. Leave the flapping to others.
Your local dive shop should be able to supply you with the equipment mentioned here. Specifications and prices vary enormously. Here's a guide.