Freediving watch

A freediving computer records each dive: how deep, how long, and how long you rested before the next one. That last figure is the one that matters. Blackouts happen to divers who dive too soon after the last dive, and the watch is the only honest witness to how long you actually rested. The rule of thumb is a surface interval of at least twice the dive time, and more as the session goes on; a watch makes it a number rather than a feeling.

It also tells you the water temperature, which tells you how long you have left in the suit, and it turns a vague "about 15 metres" into a real depth profile you can learn from.

What matters:

• A freediving mode, not just a dive mode: it must sample fast, at least once a second, and show the surface interval clearly between dives.
• Surface interval alarm: an alert that tells you when you have rested long enough, set to your own rule.
• Readability with a mask on, in low light, with wet eyes: big digits, a backlight.
• Depth alarms: useful for a diver working a fixed depth; a nuisance if they beep on every descent.
• Battery: a replaceable coin cell that lasts a season, or a rechargeable one that lasts a week. Either way, a battery that dies mid-session is the fault of nobody but you.
• Robustness: it will hit rock. Metal or a hard polymer case, a screen you can scratch without losing the numbers.
• Log: the ability to review the session afterwards is what turns the watch into a teacher.

A sports watch with a "dive" feature is not a freediving computer. A basic freediving computer from a dive brand costs about what a good pair of fins does and lasts years.

Worn over the suit, on the wrist you look at naturally. The strap must extend over 5 mm of neoprene and cinch tight enough not to slide.

Rinse the buttons under fresh water and press them while rinsing to flush salt. Replace the battery before a season, not during it. Keep the screen out of the sun between dives.