When an object is placed on top of a liquid surface or completely immersed inside the fluid, there is a force acting on the object vertically upwards against the gravitation. This force is called the buoyancy force.
The force of buoyancy is result of pressure exerted on the immersed surface of the object by the fluid particles. This force is found to be equal to the weight of the liquid that is displaced.
If the force of buoyancy is less than the weight of object, the object sinks. An example is the case of a iron ball. If the force of buoyancy is equal to the weight of the object then the object floats on top of the liquid surface (partially or fully immersed).
If the density of the object is less than the density of fluid, then the object floats. What portion of the object is immersed is decided by the density, shape and volume of the object.
The principle of floatation is used by boats, ships, submarines.
Floatation is the concept of lying on sea water on specially designed sea water tanks. These are flexible water beds, which we can place in our beds. This is a therapy. The person lying on these sea water tanks experiences weightlessness. It is supposed to cure some ailments, relax and relieve a person of anxiety and stress.
The floatation bed consists of around 25 cm thick fibre glass container with epsom salt water in it. The temperature of water inside should be near our body temperature. So we donot feel cold. Sleeping or lying on it, the muscles get relaxed and there is a feeling of floating in air. Usually a person may plug in some cotton in ears and close eyes to be in peace and isolated for some duration in a day.
So there is a mental relaxation as well as physical relaxation.