Because any solution, which sea water is, contains more than water only. Salt is the principle "contaminant' and the slight tendency of the little sodium (+) and chloride (-) ions to slow down the water molecules by electrical attraction at the molecular level, slightly more energy [higher temperature] is required to release the the water. FYI as you boil seawater down and the salt concentrates a higher and higher temperature will be required to "release" the water.
coz of the salt
The vapor pressure of the water is decreased because the water is effectively diluted by the salt. Because the vapor pressure is decreased, the water must get hotter in order for the vapor pressure to equal the atmospheric pressure and therefore boil.