I had swum all my life and assumed that would carry me. It carried me about as far as the surface.

The instinct you have to unlearn

Underwater, almost every instinct you own is slightly wrong. Something drifts out of reach and you kick towards it, which changes your depth, which changes your buoyancy, which means you now kick harder. A regulator feels odd in your mouth and you tense your jaw, which burns air. Silt clouds up and you turn to look, which puts your fins where the silt was.

The whole first course is a long argument with your own reflexes. Instructors do not say it that way, because "stop reacting" is not a skill you can practise directly. So they teach it sideways: hover here, do nothing, breathe.

Breathing is the control surface

Nobody tells you that your lungs are the primary trim adjustment. Inflate a little and you rise; exhale and you sink. Once that lands, you stop treating the inflator as a steering wheel and start using it the way you would use a thermostat — a few times a dive, not a few times a minute.

What actually goes wrong

Not sharks, not equipment, not depth. It is almost always the same three things: going too fast, going too deep too early, and being unwilling to end a dive. Every one of those is a decision made on the surface, before anyone got wet.

The best advice I got was from a divemaster who had watched me thrash through a hover exercise. He said: you are trying to hold still. Try being still instead. I have since heard almost exactly that sentence about debugging.