Why does the composition of Ocean Water list sodium and chloride separately?
First of all, of course you could provide NaCl content as a weight or molar percentage, and I can demonstrate if necessary. 

However, this doesn't really reflect the reality: ocean water is a solution of many salts. Chloride does not necessarily only come from NaCl, but can also come from KCl, MgCl$_2$, and so on. All the ions are solved individually, and if you start evaporating the water, you will get a mix of salts. 

So there's really no point in showing NaCl percentage. 

Of course, you're right in that it is impossible to obtain pure chloride ions. But the chlorides you will obtain will not necessarily be those of sodium.