Because our lungs act as a air tube in living body
There are a couple of reasons. First, when living, we move our arms, legs, neck and everything. This motion causes us to sink. IF you recall swim lessons when you were young, the instructor says lay still. If you lay still, you will float but the second you begin to move, you will sink. An unconscious person will float but has a tendency to float face first which in turn will drown them. Second, when you die, you begin to bloat, this bloating also causes you to float but once again, if you research it, you will find that most dead bodies found it water will be face down.
Your body after dead became lighter than we living so it float I mean lighter than water...
Because the dead body starts too decay and loses weight in the way of no oxygen non of the organs are working , so the body becomes light, but with a live human there is oxygen been brought in and lungs, and heart pumping and all internal organs are safe and away from loosing weight/decaying so that is why m8 I might be wrong but that is what I think :S
When your lungs are filled with air them you will float to the top of the water but when you release the air in your lungs then you will begin to sink.
A dead body can float on the water,because its density is bigger than water's.