We have had many threads on SB about LIFE SAVING GEAR....Everything from Epirbs to Nautilus Lifelines, to inflatable dive rafts, and on and on.....
So imagine you were 3 to 10 miles off shore, and had the expectation that suddenly, you would be out here on your own for many hours, maybe even a 24 hour day till someone found you....I am not talking about planning for something like this, I mean, out of the blue, it just happened..the impossible....
Freedive skills and a Freedive snorkel are essential lifesaving equipment for this. No other special gear is needed, for you to survive for this kind of duration..just the skill to use them at a freediver level..on the surface.
You might even remove the bc and push it in front of you, and have the tank as something you have to push in front of any curious sharks, that just need a solid object to get a reaction from, over the many hours you are drifting in deep water. As long as you can remain awake, as long as your exposure protection ( choice of wetsuit or dry suit for day) is keeping you warm, and if you have some moderate fitness, you don't need anything else....
I would call this pretty damn cheap insurance...just having to learn freedive level snorkeling skills, and getting the less expensive simple J snorkel.
Another plus, would be if you had a compass ( you know you should) , then you could be slowly swimming in the direction of shore the whole time, breathing through the snorkel.
Or, the more likely scenario, you are just far from the boat, low on air, and you just swim toward the boat, with snorkel and compass, looking up for boat every couple of minutes. Facing away from the boat, kicking on your back, is not the best way to find the boat....and a poor way to swim to shore also, compared to the efficiency of a flat horizontal swimming position on top of the water, breathing through the snorkel....When the boat is coming to you, the swimming on the back is fine.....and the boat IS coming to you anyway, most of the time, right?