From what I understand it wasn't the force of the water that did damage, it was simply a rise in the water level of about 15 feet which very quickly flooded the area and carried things with it. If you had on a life jacket you would likely have survived the tsunami if you didn't get caught under something.
No. In most cases a lifejacket would not help. The vast majority of victims did not have time to drown. They died of trauma injuries, from being swept at high velocity through buildings, rocks, poles, trees, cars etc. They were crushed, impaled, cut in bits, massively lacerated. Very few died 'peacefully' drowning. They were swept into sharp unyielding objects at high speed and killed. People on site say the victims look like they died in motor vehicle crashes.
Sure, a lifejacket might have helpes a small % of cases. But not many.
Neither the in-rush nor the out-rush gives you much chance to bob up buoyantly. In the former case, water is piling up over you, pushing you down and forward, scraping you along the bottom. In the latter case, it's the undertow from Hell and you're probably dead by then anyway.