There are several salts in seawater, but the most abundant is ordinary table salt or sodium chloride (NaCl). Sodium chloride, like other salts, dissolves in water into its ions, so this is really a question about which ions are present in the greatest concentration. Sodium chloride dissociates into Na+ and Cl- ions. The total amount of all types of salt in the sea averages about 35 parts per thousand (each liter of seawater contains about 35 grams of salt). Sodium and chloride ions are present at much higher levels than components of any other salt.
Reference: DOE (1994). In A.G. Dickson & C. Goyet. Handbook of methods for the analysis of the various parameters of the carbon dioxide system in sea water. 2. ORNL/CDIAC-74.