The counting unit for compounds is a mole, i.e., one mole of NaCl is made from combining one mole of Na and one mole of Cl. The weight of a mole of a compound is simply the sum of the atomic weights of the atoms. Any element or compound that has an electrical charge is an ion. We tend to describe dissolved chemicals in seawater as dissolved ions because we can use the term to describe elements and compounds at the same time and almost everything dissolved in seawater has an electrical charge. Cl - is the chloride ion since it has a single negative charge.
Compounds can be ions, too.
In practice, the charges are often left off when we write down the formula for an ion.