Nearly all floatation devices rely on air to keep them afloat. Poke a hole in a life raft, for example, and the chances of a dry rescue escape along with the leaking air. Likewise, a ship, regardless of its shape, couldn't float without the air that fills the open part of its hull. In fact, all air, whether trapped in a life preserver or not, floats on water.
Air floats because it is less dense than water. This means that a given volume of air weighs less than the same volume of water. Indeed, water is nearly 1,000 times denser than air. Any object or substance that weighs less than the amount of fluid it displaces will float on that fluid. For example, if a one-cubic-centimeter object weighs less than one cubic centimeter of a fluid -- the amount it would push out of the way when submerged -- it will float on the fluid. If it weighs more than the same amount of fluid, it will sink.
All objects, whether they sink or float, experience an upward force when submerged in a fluid. This force, called the buoyant force, is equal to the weight of the fluid displaced by the object. For an object that floats, the upward buoyant force is greater than the object's weight. In fact, the buoyant force allows low-density objects to not only float, but also causes them to resist being forced under water and to rise quickly to the surface when they are released. A container of air, for example, experiences a powerful upward force when placed under water. When air bubbles are released, they respond to this force and race quickly to the surface.