The odds are that your packs are at least as buoyant as you are, unless they are loaded with scrap iron and water. I have been in one situation where an on body psk does make sense - flying on a relatively small charter aircraft. In most of these, your pack, even a small day pack is placed in a cargo compartment. in case of an unplanned landing and exit, you would have only on body items for immediate use. I usually just beefed up my EDC