The life vest is self-inflating on contact with water (possibly salt water, I don’t remember), built into the the harness used to “Strap in” to the jet, and designed to roll the aircrew face-up in the water. Or at least that’s the way things worked when I flew in fighters circa 1988-1992.
Part of our annual air crew training included water survival. The sucky part of this was getting dragged behind a boat to simulate wind drag, and having to release the parachute harness. The fun part was we got to go para-sailing at the end of training.
I also got to experience the infamous “dunker” training for chopper crews. Did. Not. Like. Picture a big ass oil drum on steroids. Big enough to seat a crew of six or so. Suspended over a swimming pool. Capable of dropping into the pool, slowly sinking, and then rotating upside down. We had to wait for the damn thing to sink, trying to grab one last good breath before it did, then wait for it to flip over, and finally de-ass the drum and swim to the surface. After we passed that exercise, we had to do it again. Blindfolded. It sucked.