Take eight divers and subtract two. You’re left with six, right?

And that’s the simple arithmetic that the crew of a luxury-resort speedboat couldn’t manage. They returned to shore with only six people.

Across the table at a London pizza-house, I sat interviewing Olga and Robert, the pair abandoned off the coast of Cuba.

That they had survived was down to luck. A fisherman had spotted them bobbing in the water three hours after they’d surfaced.

In the packed restaurant, there were no raised voices complaining about wrong dishes from any of the busy tables surrounding us, because the waiters had logged the orders. Olga and Robert were not logged out of or into their boat, and were not missed.

The first the dive-centre knew of their clients’ near-fatal ordeal was when the couple walked in and told them.

My resulting feature, Missing, was published in 2003. It reviewed a handful of incidents in which divers and boats became separated. I’d worked on the story for two years. That the article was so delayed was partly down to a long-gone Florida dive-operator refusing to discuss how it also forgot two of its customers, who spent 24 hours on a light-tower before being spotted by a yacht.

A lawsuit followed and sentencing required the operator to share advice on how to avoid a repeat with anyone interested. I thought divEr readers would be very interested, but the company wasn’t sharing, so my report stalled.

Exasperated, and with the imminent release of Open Water (the fictionalised film inspired by two divers left behind and never found off Australia in 1999) the Editor pushed me to finish the piece.

He also pointed out the topicality of a dozen divers going missing from their liveaboard in the Red Sea just as the movie was hitting the cinemas. The truth is, however, that when it came to such incidents, I was spoilt for choice.

And, all these years later, I still would be.

Causes of Separation

Divers get separated from boats for many reasons. They don’t always follow briefings, and surface in the wrong place, or get caught in unexpected current and swept away.

A skipper makes a mistake, or the boat breaks down. If the boat has a proper accounting system, the crew know that they have divers missing and should get searching quickly, but this isn’t always the case.

In some incidents, boat-crew had no radio to summon help. In others, they delayed reporting the incident for hours, hoping to find the divers themselves. In that time a diver can drift for miles, and night can fall. A diver’s head is a tiny target, even under ideal sea conditions (“you’re looking for a cabbage”, a sailing instructor told me).

Flags, DSMBs and signal mirrors can help searchers; an air siren might be heard at short distances over quiet engines. But luck plays a scarily big part in being found. The worst scenario, as with Robert and Olga, is not to be missed at all and have no-one looking for you.

The Product

The Nautilus LifeLine is one of the most important safety aids ever made for divers. It’s a personal location beacon (PLB), a radio transmitter that can signal for help and reveal your exact position to a searching boat or aircraft. It empowers you to do something positive towards surviving a separation.

PLBs are not new, and the LifeLine itself has been around for some years, but the version I tested is a new model although, as I’ll explain, it isn’t necessarily an upgrade.

It was developed by Mike Lever, owner of well-known liveaboard Nautilus Explorer, following a scare. “The Nautilus LifeLine was the culmination of spending 20 years in dive skiffs in the chilly, current-swept waters of British Columbia and Alaska,” he explains. “Wondering. Hoping that all my divers were going to surface where I expected them to.

“I always worried about losing a diver and then one day, off Wooden Island in the Gulf of Alaska, one of my divers didn’t surface when they should have. We started searching. The current was increasing as we got further from slack. I tracked out into the open gulf in a 6-8ft swell with no joy.

“I called the US Coast Guard for help and they told me that it wasn’t an emergency yet (in other words, piss off and don’t bug us until it’s too late). It took us 50 minutes to find the diver drifting several miles from shore.

“We were all in tears – except for the missing diver. He was calm and fine, because he had seen us searching for him and assumed that we would find him.”

I have helmed small boats but have no formal boat-handling training. I see boats as either taxis or wrecks so, to help test the LifeLine, I turned to Nick Balban, a diver of some 25 years experience, BSAC instructor and former Gibraltar SAC Diving Officer. He is professionally qualified to skipper vessels of up to 24m.

The Design

The LifeLine’s electronics are integrated into a compact polycarbonate housing that’s watertight to 130m. A single catch, easily operated with a gloved hand, lets you open the O-ring-sealed lid on the surface.

This exposes the aerial and three push-button controls. These parts are waterproof, but not pressure-resistant. Two Philips-head screws give you access to the battery chamber, which takes two CR123 cells.

LifeLine displays position advice on chart plotter . This lets boat see the diver’s position without setting off a rescue alarm.

The LifeLine is not an electronic positioning indicating radio beacon. EPIRBs work over much longer distances, by relaying their signal via a satellite, alerting call-centres that can co-ordinate a rescue. Standard safety equipment for operating far from land, they are found on aircraft and yachts because they can summon help anywhere in the world, even though that help might take days to arrive.

The LifeLine transmits a radio signal limited by both its power and antennae, and has a claimed range of 34 miles. In many diving locations, the closest and fastest responder is likely to be your own or another dive-boat, given the lack of search and rescue infrastructure and time needed to deploy the device in many destinations. So the range should suffice.

Once you’ve flipped open the lid, removing a slip-off safety guard allows the aerial to automatically uncoil (watch that it doesn’t flick into your eyes) and reveals the red emergency push-button control. Hold this in for five seconds to send out a distress signal.

It attempts to contact suitably equipped craft within range and, if it succeeds, your GPS co-ordinates will be displayed on their navigation instruments.

There’s a 20-second window between pressing the call button and the unit transmitting its emergency signal, during which time you can turn the unit off in case of accidental operation.

Along with the distress button, two other buttons control the LifeLine. These can be accessed without removing the cover, which is basically a safety lock and should, along with the five-second press, prevent misfires.

One is the on/off button, the other a test button, used to check that the battery is functional. Waterproof user instructions are printed on the unit.

The previous LifeLine allowed you to talk to rescuers via a built-in walkie-talkie. I can see how reassuring this could be, because the current model has no way of indicating that your distress signal has been received and is being acted upon.

However, in some parts of the world a radio licence was required to use the LifeLine so, for practical reasons, it’s been dropped to remove that barrier to ownership. This is why the change in specification isn’t an upgrade, even though the electronics have been improved.

What is a major improvement is that the LifeLine now has a function that broadcasts your position, not as an emergency alarm but as an advisory measure.

The psychological benefit of this option is that it removes the conflict divers might feel if they’re concerned that they have not been seen, but are reluctant to set off a full-blown search until sure that they’ve been lost or abandoned!

For me, that’s a huge selling-point.

The LifeLine shows distance and heading to a diver at the surface.

The LifeLine is now basically an AIS (Automatic Identification System), commonly used on ships to prevent collisions by making them visible to other sea traffic. Each AIS is specific to the vessel and you can check the ship’s name, tonnage and other details online.

Set off the LifeLine’s AIS function and your position is indicated by a numbered icon on the screen of the boat’s chart-plotter, allowing each individual LifeLine to be identified.

In theory, in advisory mode the AIS function can also be linked to your boat’s radio, so it automatically sets off an ear-splitting alarm that can be silenced only by acknowledging it.

Otherwise, the on-screen display is passive – someone has to notice it.

Again, to me, the unignorable audible alarm was a persuasive selling point, knowing how lax “cover” can be. However, for unclear reasons, legislation makes this safety feature inoperable in European and some other waters, and so it proved in Gib.

Pairing the LifeLine with your boat requires use of a free app and is straightforward.

It can be used to train on the unit or, as Nick and I did, to get a feel for how it works from the boat-crew’s perspective by checking the helm instrument read-outs.

Because the LifeLine’s capabilities depend on local regulations, and these affect how it should be programmed, it’s essential to check the manual before use, especially as a dive-traveller.

As a guest on a dive-boat, you’d need to ask the skipper to agree to pair your unit, but it’s difficult to see any reasonable objection to doing this. You should then follow the crew’s instructions regarding using the LifeLine.

Depending on where you are in the world and whether you have paired your unit with your boat, the LifeLine might first try to alert only your vessel of your location before, 30 minutes later, sending out a distress signal to all ships within range and able to receive the transmission.

This will set off an audible alarm, which should be treated as an emergency by these craft. They should immediately head for the GPS co-ordinates provided by the LifeLine.

Within those 30 minutes you can cancel the transmission if your own boat has found you.

To help rescuers locate you in the dark, a white LED flashing beacon automatically switches on as daylight fades.

The LifeLine stores easily in a BC pocket, though a dedicated pouch is also available.

This has a coiled lanyard to which to secure the LifeLine, so if you fumble and let go of the unit, you’ll still be attached to it when the cavalry rocks up.

It also allows for mounting the LifeLine high on a shoulder-strap, so it’s clear of the water and you don’t need to hold it while you await rescue.