I'm at 38 m, narked and trying to lock focus on a dive computer which has just gone into decompression mode. Visibility is at best 1 m and dropping. These are the worst diving conditions anyone can remember off Gibraltar. I have a reel with me, which is tied to our shot line, but I don't want to let the line run free for fear of entanglement. I can't control it with my hands full of cameras and hand-held strobes, so I put it down, but in doing so set off a chain of events. Having obtained my images, I look for the reel to guide my partner and myself back to the shot. I've moved no more than 2 m away, but I cannot even see that far.

Nick and I were the first divers down because we wanted to be ready to photograph the computers going into decompression mode, so we've been hanging around waiting for the test team to arrive, and for their Aladins to enter deco. Consequently, we are already into decompression ourselves and our hang time is rapidly clocking up. I grab Nick and try a quick square pattern search. A current is running and probably pushing us off course.

We're racking up more and more stop time while depleting our air. I decide to ascend without finding the line, so we ascend slowly, facing each other to avoid separation. Reaching our stop we hover - I'd much prefer to be on the shot line because it makes controlling the stop level so easy. We know the current is strong, so we're drifting, but I'm more scared of getting bent than getting lost at sea, so I'm not going up to signal the boat. When we do surface the RIB is a speck on the horizon.

In the RIB holding station by the shot the coxswain is expecting all six divers to ascend up the line, but all have failed to relocate the line. Three teams hit the surface at different times and locations. We have been decopressing the longest, so we surfaced last and drifted the furthest.

I own a rescue tube for such eventualities, normally it is tied into my BCD and had a marker light (just in case), but the previous day we'd used it to mark something and I'd forgotten to replace it in my pocket - so the boat couldn't see us. We held our cameras aloft, fired our strobes and tried to reflect the sun off our lenses - no joy.

Finally, a Spanish fishing boat passed the word to our boat that we were two miles offshore opposite a place where a great white had been seen in some tuna nets.

This is a perfect example of imperfect diving. Defensive diving is about avoiding these types of scenarios by identifying potential problems and taking measures to avoid them all together, minimise the risk, or prepare to overcome the problem if it still occurs. In a word, defensive diving is about survival. Theoretically, every entry-level diver is taught defensive diving, but every year accident and fatality reports suggest this information is not taught, understood or heeded. I have a presentation for rescue courses called "Bet Your Life" - all the participants are not winners, their last dive history pulled from fatality reports. The presentation is designed to show students almost all diving accidents are a direct result of cause and effect. By analysing the reports, with hindsight it's easy to see why the accident was caused - they very rarely just happen.

But defensive diving is not about hindsight - it's about foresight - learning from other people's mistakes. Avoiding trouble isn't just possible; it's often very easy and certanly much easier than coping with problems.

The dive I screwed up on illustrates thee domino effect of one small mistake leading to others. Incidents are often caused not bby a single catastrophe, but by a knock on effect of small problems easily handled in isolation, but insurmountable in combination. Had I not lost the reel we would have surfaced up the shot line, made our decompression stops with ease and arrived next to the boat as planned.

Dive planning is really about preparation. A good plan answers: "what if?" A poor plan lets that question go unanswered until you are underwater and "what if?" becomes "what now?"

Ordinarily I would have been equipped to handle my problem and I would have had options. It's only when you run out of options that you really run into trouble.

Firstly, I shouldn't have put the reel down; if I'd rearranged my camera I could have reeled off from the shot and then clipped my reel to myself. When I lost the reel the current made my search patterns inaccurate and whilest searching I was incurring an ever greater decompression obligation.

Normally my rescue tube is attached to a small reel with 40 m of line. In fact, I could have tied off to the bottom and made a sweep search snagging the shot line. This action would probably have taken less time than my unsuccessful free-swimming search. Failing that, I could have sent up the rescue tube so that at least the boat would have known where we were. Even if I hadn't had the reel to hand, thetube would have been visible to the boat, but, we were lucky. In other articles we look at how to take the luck out of dive safety - after all, luck cuts two ways.