spare bottles

I personally think that a spare air could be a good thing, but only under certain circumstances. Using one can however be very risky.

Using spare air is scuba diving and on top of that in an emergency situation. I think to be able to use it meaningfully you should at least be certified to dive to preferably even deeper depths than you are freediving to. Not just getting a certificate but also being a confident diver at that depth. Also you should be very good in scuba emergency training and also train with the spare air itself. Otherwise you might falsely feel safe while in the mean time the spare air adds more security problems than it resolves. Some even go as far as advising never ever to use it at all because they think the risks are too great compared to the usefullness of it.

See also previous posts about spare air bottles like:http://forums.deeperblue.net/showthread.php?threadid=43222

Ascending while breathing compressed air

Without scuba diving training and understanding the dynamics of breathing the compressed air at depth, chances are you WILL KILL yourself if you attempt using compressed air for emergency ascent.

Possible consecuences are:

General Barotrauma, Raptured Lungs, Atrerial Embolism. All of which might result in your death.

Please get a proper training if you plan on breathing compressed air under water.