Good drinking water is free of ionic chemicals and is neutral.
Sea water consists of salts, impurities and other minerals. They exist in water as ions. They may have some electric or magnetic effect on the magnetic compass needle.
The sea water due to the salts in it, is slightly alkaline (pH = 8 or so).
The alkalinity of sea water does not come from Na Cl. It comes from dissolved CO₂ over many years/centuries. In sea water there are dissolved CO₂, corbonate CO₃²⁻ and HCO₃⁻ ions too. Due to these the alkalinity arises.