About five years ago,I flew with a U.S. low-cost carrier (B737) from Reno to San Francisco and noticed that there was no mention of life-jackets during the Safety Briefing. I couldn't locate "my" life-vest under the seat so politely asked a crew member where they were and was told, "You got it right under your butt honey. It's the seat cushion that you take with you if we all gotta take a swim."
Obviously it saves weight,hence fuel burn etc not carrying jackets. Please nobody tell Mr. O'Leary, it's a money-saving wheeze!! I ain't gonna swim with no life-jacket on!
A thousand times "well done" to Captain, First Officer and Cabin Crew.