DEAR SIR ,

when we are alive we cant float in sea water . but when we are dead we can float in water .

WHY IS IT SO ?

Dear student,
Once dead, the body would sink (assuming that the person did not have excessive amount of fat in the body, which are less dense than water). After a few days of sitting in the water, the water penetrates most parts of the body, thereby equalizing the density and hence the buoyancy is approximately equal to its depth. As gasses collect and the water washes away the more dense byproducts, the body becomes less and less dense until it can float on water.

In the sea/ocean, saltwater is actually more dense than freshwater, which therefore makes it easier for things to float on them.

Live bodies most certainly float on water, if they are fat enough as the Fat is less dense than water. Fat people actually float quite easily.
Whereas muscle is more dense than water, so muscular people sink in water. Of course they can take a large lungful of air to offset the sinking effect, but it sometimes isn't enough.
Regards
Sumit