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.
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).