Does this deflate?
I agree with Leni C, by the way. I think the vest is rather large for snorkeling and if it's inflated at all, it gets in the way. It's more like a security blanket for those who are not entirely comfortable in the water, or to allow someone to float and rest a while. When inflated, it would make me feel claustrophobic. That said--it serves the purpose for which we purchased it, but would have probably been okay ordering the "youth" size.
Yes. It deflates if you hold down the valve and probably would want to press on the vest to help speed up the process. It appears that you can lock the valve in place so you do not accidentally deflate it (though I don't think it would be easy to deflate by accident).
We bought these because we live in Bermuda and are often having visitors who want to snorkel but do not feel all that comfortable in the water, so the vest gives a sense of confidence, without getting too much in the way when it's deflated. I don't think it's meant to be a life jacket for a boat or something like that, though.
This vest only inflates and deflates manually. There is no co2 cartridge for emergency inflating which I would have liked. This vest is enormous which is unnecessary for snorkeling and in fact interferes.