Inherently buoyant

A foam vest floats because foam floats. There is no cartridge, no trigger mechanism and no expiry date on a canister. Put it on and it is working, including if you are unconscious when you hit the water.

That reliability is why it is the only kind approved for anyone under sixteen, and why it is the sensible default for beginners, kids and anyone who is not going to maintain equipment.

The cost is bulk. Foam takes up space, and a foam vest is warmer and more restrictive than an inflatable belt pack. Fishing-cut foam vests reduce that with ventilated panels and high-back designs.

Foam does age. Closed-cell foam compresses and absorbs water over many years, so a vest that has been crushed under gear in a locker for a decade floats less than it used to. Squeeze it: if it feels hard, flat or waterlogged, replace it.