Will seawater kill weeds

Yes, seawater will kill weeds if the plants in question are salt-intolerant. Weeds may be found growing around and in brackish, fresh, salty, and stagnant waters. Intolerance and tolerance will depend upon the plant in question's ability to handle waters hugely or slightly different from the norm and to have all or some of their component parts immersed in water since many do not tolerate waterlogged roots.