I think you'll find the Hudson aircraft did have lifejackets onboard. Most of the passengers probably just didn't put them on- after all, the FAs said they had no idea they were going to ditch until it happened. Many airlines only train the FAs to call for lifejackets in a planned ditching. Others may have looked out the window and seen that they were on the river and figured they wouldn't need a lifejacket.
See image here:
http://news.uns.purdue.edu/images/20...ger-hudson.jpg
Wodrick, unless a body is trapped in wreckage it will usually float after 5 or 6 days. If you mean making them easier to spot, well, it would, but that's not the reason they are there...
The lifejackets onboard the Ethiopian 767 were the old type where you put your arms through the strap and quite difficult to undo since they had a one-way tightening strap, not a plastic 'click' buckle like they do now. Many pax simply could not get them undone when they floated to the top of the cabin.