Naturally sea water contains NaCl (sodium chlorite) , which we called as common salt.But in rain water doesn't contain any gases or other materials in it. Sea water is useful only for survivallance of aquatic animals.Thus sea water is not useful for human survivance.
   For Example:1)Sea water doesn't able to drink but rain water can be drink.
                       2)We can construct dams and use water for growing of crops and                             other things but sea water unable to use for growing of crops.
Sea water is salty,which is unfit for the growth of contains different types of salt like sodium chloride,magnesium chloride etc.its not good for drinking.rain water is the purest form of water.we have to collect this rain water for our survival