Tsunami. A tsunami's danger is not just from fast moving currents but also from all the debris it contains. Although a life jacket may offer little help - If I was stuck inescapably from the path of a tsunami - I would pray and put the life jacket on. Bottom line- don't count on a life jacket to save you.
Not much. If you are struck by the major aspect of the tsunami wave, the life jacket will be of little help. If you are at a low velocity, low volume edge of the wave it might give some help. The best way to be around a tsunami is to be somewhere else or be incredibly lucky.