It is a true reflex I practice breath holds due to the fact that I spearfish, when I started I could hold my breath for about a minute out of water and less than 2 in water. Funny part is usually you second or third hold is the longest as your body due to the reflex tends to adapt to the higher CO2 levels in it, on the second and third breath hold. I can now hold my breath in water for over 5 minutes due to CO2 desensitization. The average person with training can hold for 8 minutes without loosing consciousness if they are calm and and resist the urge to breath. I have never tried to push past 5 as it leaves a decent window and 5 minutes is a really long time underwater. I can spend 2 minutes at 100 ft with a 5 minute breath hold. I am 45 and am in average to just slightly above average shape for a 45 year old.
Part of my divemaster training included free diving training.
While I hatted it, I experienced the same thing as you did: I could hold breath for a minute out of water and 2:08 underwater. There's the preparation, getting calm and relaxed, but there's also the diving reflex playing some part there and preparing your body for longer immersion.
No the reflex does not depend on temperature. The immersion in water is what initiates the reflex. Though it is true that cooler water prolongs the reflex. I spearfish in the Caribbean so average water temps are 70s-80s.
It is the struggling and adrenaline that causes rapid loss of consciousnesses in water, as it rapidly depletes oxygen and works against the reflex. If one calms themselves and uses deliberate movement 5 minutes is achievable in 2-3 months of training in tropical waters, by an individual in average health.
When we spearfish we don't race to the bottom or rocket to the top, we use a lazy peddling motion to slowly decent and ascend. It helps in preservation of oxygen, once on bottom we usually hover in one spot, and try to remain still while almost meditating. We only move while actively hunting a particular fish once spotted. While the reflex is active your mind can work with the reflex to help it shut systems down and focus blood to the brain which is what it is doing. Each movement reactivates a system, eating before a dive activates a system, not using the restroom before a dive can leave a system active (though the reflex generally tries to shut that down by increasing the urge as soon as you enter the water, hence the have to pee when you get in the pool reflex).
When you have been in the water long enough sharks are a natural encounter. After a while they just become part of the landscape to you. They are attracted to the struggling fish and will try to bully you for your fish but rarely are you their main attraction. When you realize that they are a lot like dogs is when your attitude starts to change with about them being around. You realize if you stand your ground give them a nudge with your spear, they generally loose interest really quickly. They avoid fighting for or with their meal. They always say it is the shark that you don't see that will get you and it is true. As long as your keep them in your sight, and confront them they will loose interest. I am no more afraid of a shark nowadays than I am a dog. You give them respect but you stand firm. I have only ever been in one really hairy situation and that was with a persistent 14 ft Tiger shark. Long story short after several nudges with my spear, and it just circling back around, I used what we call a blowfish defense on it, where you ball up and as they are coming in you stretch out your body, arms and legs quickly. That was enough to startle it and cause it to loose interest. That was the first and last time I have ever had to blowfish, All of the rest where easily persuaded that I was going to fight for my meal.
