if a normal person dropped into the water he may go under the water then why if a dead body dropped into the water mean he may float why
People tend to have positive buoyancy when alive as their lungs are full of air. This makes a persons total density less than that of water. This made obvious by the fact that scuba divers usually wear lead weights . A persons physical makeup can also influence this lower density - more fat, less overall density.
External to the body itself some folks carry a great deal of ballast (camera, coins, tool belts, heavy boots, outboard motor accidentally caught in clothing) which increase their average density and they sink.
A dead body (at firat) floats or sinks the same as when the person was alive as nothing changed - density wise. However the lungs may fill with water either b a last gasp inhales water or by puncture wounds to the lungs (e.g. a car accident) This increases the density and the body sinks.
Then comes the miracle of decomposition. Microbes eating the intestinal contents generate gas which is held in the intestines. This increase the buoyancy and the body rises up through the water.
As time goes on, leaks appear in the tissue holding the gases in (fish, crabs, wear and tear) and the gas escapes, increasing the average density and the body sinks for the final time.
Many gases produce in body of human. When the man die the water enter the human body and there is no way for air to diffuse out of the human body .As the gasesare lighter so they try to diffuse out of the water due to this the dead body floats on water