Chloride ions for the salty seawater
Chlorine is a very reactive element. Contrary to your statement, chlorine occurs in many rocks and minerals.

The mineral apatite, with the composition: Ca5(PO4)3(F,Cl,OH), is a natural source of chlorine. Virtually all rocks contain chlorine.

Chlorine even occurs occurs in alkaline magmas. 

One of the most common minerals containing chlorine is common salt, which is sodium chloride, NaCl.

Geologically, chlorine is everywhere. When rocks are exposed to water and acids minerals, including chlorine, dissolve into solution and they are transported to lakes, salt lakes, aquifers and oceans.