Hate to differ with vodkaholic but on the hijacked Ethiopian 767 more people died because they inflated their lifejackets than survived becuase of it.
When the cabin came to rest underwater it was neccessary to swim down to get to the exits/cabin breaks and with an inflated life jacket that is almost impossible.
For the same reason, if you have a lifejacket that needs to be tied, always tie in a double bow as instructed. This way if for some reason it becomes inflated and you need to remove it to swim down you are able to. If its tied in a knot that may not be possible.