Question

why do a dead body floating On The Water surface?

Answer 1

Initially, bodies will likely sink to the bottom, but as bodies decompose and gasses are released in the body, the corpse will rise back to the surface as a result of buoyancy.

Answer 2

Things float in water if they are less dense than water, i.e. they weigh less than the same volume of water. The density of a human body with a lung full of air is about 0.985kg/l - slightly less than water (1.0kg/l) - which is why, if I throw you in a swimming pool, you'll float with a small amount of you (0.015 by weight out of the water). If you don't hold your breath because you panic or because you're dead, your weight remains the same, but your volume is smaller. Your density therefore increases to - about 1.08kg/l. This is higher than water so a dead body will actually sink.
OR
Our body has many bacterias mostly in our torso region(mostly intestines) . When a Dead body goes inside water or is dead inside the water Our Lungs which initially is full of air, get filled up with water. Simultaneously the bacteria about which I mentioned earlier starts producing various gases for example “METHANE GAS, CARBON DIOXIDE”. Such gases produced makes the density of our body lower then water and our body starts acting like a billions full of gases AND as we know that ,if the density of water is greater then any body Then that body starts Floating on water. This is the reason for the floating of Dead body in water.