Airlines don't advertise it, for obvious reasons, but anyone inflating their life-jacket, before being "in the clear" is signing their own death warrant. You will simply float to the top of the cabin, then drown, as I believe many did aboard the Ethiopian Airways flight that ditched a few years back.
Only inflate your life-jacket when clear of the aircraft; you're much better off being a good swimmer, but the life-jacket will come into it's own if you have to spend any time in the water. The point is, there should be no rush to inflate it.