Question

a person did not knew how to swim and he was sink after die he came on the surface of water why he come on the surface after die

Answer 1

because his dead body started floating on water and was left on banks I.e surface
hope it helps

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.