Why cant human beings live in water?
Humans can not live underwater because they can not breathe underwater. The reason we cannot breathe liquid water, because the oxygen used to make the water is bound to two hydrogen atoms, and we cannot breathe the resulting liquid.
Fish "breathe" the dissolved oxygen out of the water using their gills. The main reason why gills work for fish is the fact that fish are cold-blooded, which reduces their oxygen demands. Since humans are warm-blooded animals it would be hard to extract enough oxygen using gills.
Humans cannot breathe underwater because our lungs do not have enough surface area to absorb enough oxygen from water, and the lining in our lungs is adapted to handle air rather than water.