Defensive Diving Part 2: Deviance & Depth


Defensive Diving Part 2: Deviance & Depth

The diving set-up doesn’t seem ideal but you go ahead anyway and get away with it. Perhaps it was OK after all; perhaps you were just lucky. SIMON PRIDMORE takes it from there

0119 technique early morning

Early morning above a reef wall – there is no need to go deeper than the well-lit zone.

IN THE FIRST PART of this short series, I made a correlation between scuba-diving and driving a car, particularly in the context of learning how to anticipate and assess dangerous situations, make well-informed sensible decisions and stay safe. These are things that motorists tend to group together under the catch-all phrase of defensive driving.

Here are a few more strategies that I see as intrinsic to the defensive scuba-diving concept, where “defensive” carries the same meaning as in the motoring world, that is: safe, careful, conservative and thoughtful.

Appeared in DIVER January 2019


In a nutshell, normalisation of deviance is having a safe procedure, then cutting a corner on this procedure and continuing to cut the same corner until it becomes routine – and you rationalise that the corner-cutting makes sense.

For example: you take two lights on a night-dive because, if one fails, you can use the other. If you don’t have a second light, you’ll be left completely in the dark.

You won’t be able to see where you’re going and, when you ascend, you’ll have no way of showing your surface support where you are.

One day, you find yourself preparing for a night-dive and find that you have only one functioning light. The other has been smashed in transit – perhaps a cylinder fell on it.

You proceed with the dive anyway. You tell yourself that, if your single light were to fail, you could always team up with another diver, borrow their second light or just swim around with them, letting them light the way, then surface together.

Nothing goes wrong. Your single light works just fine, and you use the same rationale, now backed up by experience, to justify not investing money in a replacement second light. You continue to night-dive with only one light.

This is normalisation of deviance – unacceptable behaviour becoming routinely acceptable.

It is not just a scuba-diving phenomenon, of course. It appears in pretty much every field of human activity.

Nor is it merely an individual phenomenon – it can affect or infect an entire community. And it does seem to be something at which we scuba-divers excel.

Think about some of the things that scuba-divers do all the time. Recreational divers continue to dive when they are very low on air, and regularly go into deco on a single cylinder.

Technical divers often dive in overhead environments without a back-up buoyancy device; carry insufficient open-circuit bail-out when using a closed-circuit rebreather, or extend the life of CO2 absorbent beyond manufacturer- and industry-recommended limits.

It’s a matter of mindset. Having bypassed an established procedure and got away with it, some divers will argue:

“I didn’t come to harm, nor did I find myself in danger, therefore the procedure must be unnecessary or exaggerated.”

Or else they say: “I didn’t come to harm, nor did I find myself in danger, therefore I must be special in some way – some kind of diving superhero.”

A defensive diver will take the opposite view and say: “I made a mistake and I got away with it. That will never happen again. In fact, what can I do to make sure it never happens again?”


This might sound an obvious piece of advice but it is very common for divers to go deeper than they need to do, particularly at the beginning of a dive.

Only go deep if there is a purpose to it. After all, the deeper you go, the greater your uptake of inert gas, and the more inert gas your body will eventually have to discharge.

For example, you drop into the water to do a wreck-dive, expecting to be above the wreck. You look down and there is no wreck in sight. The seabed is at 30m and you know that the wreck rises 15m above the seabed. The visibility is very good. If the wreck were there, you would see it.

You look at your guide, who is descending below you. He is pointing into the distance. You look and you can’t see anything, but you guess that the guide is indicating the direction in which the wreck lies.

0119 technique Only go deep if
Go deep only if there is a purpose to it. Image: Andrey Bizyukin

You are at a depth of around 10m, having stopped descending when you noticed that you were in the wrong place. The guide is below you, close to the seabed, but still well in view. What do you do? Do you drop down and join the guide?

No, the best thing is to stay at around 10m and swim in the direction the guide is indicating. The deeper you go, the more air you consume, the more no-decompression time you use up, or the more of a decompression burden you accrue, and for no purpose.

If the guide is right, in the prevailing conditions you will see the wreck when you get close to it, and then you can drop deeper to explore it.

If the guide is wrong, you have not wasted time and air at depth and have plenty of time to try to locate the wreck in a different direction.

Guides should know this too, but if they choose not to exercise commonsense and go deep for no reason, this is not your problem (though it might become your problem if they run low on air or go into deco, or both).

Nor should you feel obliged to follow them just to keep them company. And, if there is a good reason for them to have gone deep early, they should have mentioned it in the dive-briefing.

Of course, if conditions change and visibility drops, so that you begin to find it difficult to follow the guide, and fear that you might not see the wreck from your current depth, drop deeper.

Another circumstance in which you often see divers going deeper than necessary is on an early-morning dive on a slope or a reef wall. The angle of the sun at that time of day is such that only the top section of the reef is well-lit. Further down, the wall or seabed is dark.

Unless you are planning to carry a light and treat the dive like a night-dive, there really is little point in swimming along below the well-lit zone. There is also absolutely no need to go very deep simply because it’s the first dive of the day.

Next month I will outline a few more defensive-diving strategies.

This article is adapted from Simon Pridmore’s new book:

Scuba Exceptional – Become the Best Diver You Can Be

It’s now available in both paperback and ebook versions via Amazon and other online bookstores worldwide.


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