Most divers keep a log for about eight dives and then stop. They stop because they are writing the wrong things down — and the right things take about ninety seconds and are genuinely useful later.
It is an instrument, not a diary
The reason logs get abandoned is that people treat them as a travel journal, and a travel journal is a chore.
The useful version is a working record. It answers questions you will actually ask yourself: how much lead did I need in this suit, how much gas did that depth cost me, what was the visibility here in June, and how long is it since I was last in the water?
Write down the boring numbers. They are the ones you cannot reconstruct later.
The memorable parts of a dive stay with you for years without help. The number of pounds on your belt does not survive a fortnight.
The five things that earn their place
| Record | Why |
|---|---|
| Depth and time | Your actual experience, and what a medical team would need |
| Gas used, and cylinder size | Lets you see your consumption improving, and plan future dives |
| Weight carried, with the suit and cylinder | The single most useful line in the book — never guess again |
| Exposure protection | Whether you were warm enough, which changes by month |
| Conditions | Visibility, current, surface state, and roughly what the tide was doing |
Everything else is optional and pleasant. Who you dived with, what you saw, what went wrong and what you would do differently — the last of those is the one that quietly makes you better.
Proof that you have been diving
This is the part divers discover at the worst moment, standing on a dock somewhere unfamiliar.
Dive operators want to know your level, your number of dives and how recently you dived. A log book answers all three. Without one you may be put with a less experienced group, taken to an easier site, or asked to do a refresher before they will take you anywhere interesting — none of which is unreasonable of them.
It matters for courses too. PADI asks for at least forty logged dives before the Divemaster course and a hundred for Self-Reliant Diver. Dives you did but never logged do not count toward either, which is a frustrating thing to learn late.
This is the loop closed from which certification agency: nobody much cares whose logo is on your card, and everybody cares when you last got wet.
The medical reason
A less cheerful reason, and a real one.
If a diver develops symptoms of decompression sickness, the treating team wants the dive profiles: depths, times, surface intervals, gas, and what the previous days looked like. A log supplies that immediately, and a memory under stress does not.
Most dive computers store a profile history, which covers part of this, but a computer that has been rinsed, dropped or flooded is not a record you can rely on. A written or synced log is.
The entry that makes you a better diver
If you add one line beyond the numbers, make it this: what would you do differently.
It takes a sentence. Too much lead. Ran low early because I was fighting the current. Lost the group at the wall because I stopped for a photograph. Cold at forty minutes in a 7 mm suit. Each of those is a small correction you will otherwise make three or four more times before it sticks.
It is also the entry that turns a log from a record of where you went into a record of how you are changing, and reading a year of them back is genuinely useful. Divers who do this improve visibly faster than divers who log only depth and time, and it costs about fifteen seconds a dive.
Paper, app, or both
There is no correct answer, only the one you will keep up.
An app pulls depth, time and profile straight off most computers, which removes the commonest reason people stop logging — the transcription. It also makes the numbers searchable, so a question like “what did I weight myself at last time I dived this suit” takes seconds.
Paper is easier to hand across a counter, does not need a charged phone on a boat, and holds stamps and signatures, which some operations still like. Plenty of divers keep both: the app for the data, a paper book for the signatures and the good ones.
Scuba Diving Magazine has a piece on getting more out of a log book if you want ideas beyond the basics.
One practical note on apps: export or back up occasionally. A log that exists only inside an account you lose access to is not much better than no log at all, and dive logging apps come and go.
Starting again after a gap
If you have not logged anything for years, the temptation is to reconstruct it. Do not bother.
Start from the next dive, and write down the five things. Within a season you will have a working record of your own weighting and consumption, which is the part that actually improves your diving. The gap itself is useful information: it is how you know whether a refresher is the honest answer before your next trip.
And log the dives that went badly, not just the good ones. The dive where you were cold, overweighted and out of gas early is the one with something to teach you, and it is the one everybody leaves out.
Frequently asked questions
Do I actually need a log book?
You are not required to keep one, but it is what proves you have dived recently. Operators may ask, and a diver who cannot show recent dives can end up in a less experienced group or asked to take a refresher first.
What is worth writing down?
Depth, time, gas used, how much weight you carried, what suit you wore, and the conditions. Those five make the log a working instrument. The rest — what you saw, who you dived with — is the part you will actually enjoy rereading.
Paper or an app?
Whichever you will keep up. An app fills in depth and time from your computer automatically, which removes the commonest excuse. Paper is easier to hand to a stranger at a dive center.
How does a log help with weighting?
It stops you guessing. Write down the lead you used with a given suit and cylinder and whether it was right, and you never repeat the same overweighted dive twice — see how much weight you actually need.
Do courses require logged dives?
Some do. PADI’s Divemaster course asks for at least 40 logged dives and Self-Reliant Diver for 100. Certifications you cannot evidence with logged dives are worth less than the card suggests.
Get back in the water
The best reason to keep a log is having dives to put in it. Ask us what is running locally this season.