btw I wonder why there's no such a device with a built-in computer: it would sense/analyse the depth profile on ascent, and if the freediver or a spearo would blackout i.e. re-dive immediately after a lengthy dive or not even reach the surface, the safety device would inflate a west or something. Should not be that difficult to engineer. It won't protect against a squeeze, but would be o.k. for shallow water blackouts.
There is Andy, it's called the FRV (freediving recovery vest or so) that has been advertised here on several threads. I think it's a great tool, but it's high price point (~ 1000 $) surely prevents it from widespread use. Of course I know that your life should be worth that much, but a lot of people don't have that money just laying around and I understand that it's inventor needs to cover his expenses for a low-margin product.
I was talking with Georgina Miller on the comp last Sunday and what she told me about PADI was really shocking. If you are a Scuba instructor and dive to 30m(?) plus 3 min(?) breathhold (whatever the numbers, they were way too low) you are automatically a freediving instructor. Additionally, all these courses that take you in three weeks from zero to instructor I think are absolute madness. I would never do a course with such an instructor but not all of us have this insight and imho this does not foster a safe approach to freediving. So what Harry said above I can only support but unfortunately in todays fast and goal-oriented world a lot of people don't take their time to learn something like freediving with the care and time it requires.