Why sea water conductor of electricity?

Seawater has a great deal of dissolved salt( 3.5% by weight), making it a decent conductor of electricity.

Salt water is a pretty good conductor of electricity. Its not as good as a metal but still is far from being an insulator. Pure water does not conduct electricity well at all.

The reason salt water conducts electricity is because the NaCl (sodium chloride, aka table salt) breaks apart into a positively charged Na+ and a negatively charged Chlorine Cl-. When voltage is applied, the charged ions feel the force of the electric field created and they move. Instead of electrons then, the current is the moving ions. The more salt you put into the water, the better it conducts electricity. (Pure water is actually and poor conductor because the only ions it has are the residual H+ and OH- that form in neutral water and are the reason we say water has a pH of 7.)