Doubling or even tripling up on equipment on which safety depends is becoming common practice in diving. Usually back-up equipment includes air systems, masks and lights. An area commonly overlooked, and by no means the preserve of the adventurous or technical diver, is the surface marker buoy (SMB).

SMBs perform a number of functions and are deployed in different ways depending on the dive. They serve to warn off boat traffic, indicate the diver’s position and provide a station from which to hang during safety or stage decompression stops. Sometimes they are towed on the surface throughout the dive, on other occasions they are sent to the surface when the diver is ready to ascend.

SMBs are simple devices,yet they can still go wrong. They can deflate, get punctured, be snagged by a boat, fail to inflate properly, capsize and sink, or explode on the ascent. Reels can jam during deployment, leaving the diver no choice but to jettison the buoy to avoid an uncontrolled buoyant ascent.

During a decompression dive off Gibraltar as part of a a recent series of equipment tests for Dive International, James Davidson discovered just how easy it is for an SMB to fail.

As he reached the level for his decompression stop, the gate on the reel, which had not been properly manufactured, (not by MGE) sliced through his line. This left James with 15 minutes of decompression to do in a 2-knot current, while his SMB danced across the Atlantic in the breeze.

An unmarked diver who cannot safely surface can easily be lost. An alternative to the bulk of a second SMB and reel for emergency use is MGE’s MarkaBuoy. This is a quick-to-use, self-powered, delayed SMB and line. The MarkaBuoy consists of a 32 x 20cm flame-orange buoy, CO2 cartridge and line. The whole unit fits into a pouch, sealed by Velcro and equipped with a security clip. To use the device, simply open the pouch and pull the tag. The float deploys instantly and spools off the line automatically as it ascends.

Remember to unclip the pouch before use so that any problem on deployment does not result in an uncontrolled ascent, with you being dragged to the surface unable to ditch the SMB. And likewise, ensure the pouch is clear of any possible entanglement with your kit or your buddy’s, before you deploy.

Make sure before using it that there are not any other possible snags or that you are not using it beyond the reach of the line (about 30m).

On the surface the SMB is easily seen by boat cover and remains inflated so that it can be held at arm’s length to increase visibility, unlike open-bottomed SMBs.

The MarkaBuoy can also be used to mark finds or as a lift bag, depending on depth. On the surface is has sufficient buoyancy to support a diver.

It would be sensible to check every now and again that the detonating mechanism is in good working order and hasn’t become clogged with debris, and that the cartridge hasn’t been damaged and corroded.

The gas cartridges are replaceable and the line can only be used once – spare spools are available. I had no problems with airport security taking half a dozen cartridges overseas with me.

The MarkaBuoy is an extremely valuable safety aid for divers who normally use SMBs and especially useful to those making drift, offshore or decompression dives.

• Available from most dive stores for around £35, spare cartridges cost £3 for two and line spools £4.50 for two. It is manufactured by MGE Sub-Aqua. Recommended.