Any conversation about the chemistry of seawater must begin with polarity. Without water’s unique properties, life as we know would be impossible. If water wasn’t polar it would be a gas at room temperature. This would make it impossible for life as we know it to exist at the temperatures experienced on Earth’s surface. (Ref 1)
Before I continue, this seems like a good time to define exactly what polarity is. Polarity has everything to do with the electron. In order for chemicals to react, they must share or transfer electrons. If electrons are transferred, an ionic (charged) bond is formed. This is what happens when a metal and a nonmetal interact. Most often we call the formed compound a salt. An example of an ionic salt would be Sodium Chloride (NaCl). Whether or not an electron is transferred is determined by the affinity of an element for the electron. Electron affinity can be otherwise known as electronegativity. You can imagine this as how strongly an atom ‘pulls’ on an electron. If a highly electronegative nonmetal combines with a metal that has a low electronegativity, the electron will be completely transferred to the nonmetal.
In water an oxygen and a hydrogen are in a tug-o-war for a pair of electrons. Since they are both nonmetals, neither has a high enough electronegativity to completely take the electron pair from the other. This means that the electron pair is shared between the atoms. This is called a covalent bond.
Even while the oxygen and hydrogen atoms are sharing the electrons they don’t share them equally. Oxygen has a much higher affinity (attraction) to electrons than hydrogen. So while oxygen is unable to tear the electrons away from hydrogen, it is able to pull them closer to itself. What this means is that the electrons spend more time near the oxygen than the hydrogen. The result of this is that oxygen gains a partial negative charge. Conversely, hydrogen gains a partial positive charge. This separation of charge results in something called a polar bond. This means that the bond has a positive and negative pole (like a magnet).
Now, a polar bond does not a polar molecule make. To truly be a polar molecule, the total molecule must have a negative and positive pole. Since water is a bent molecule, the negative and positive regions can be separated and this makes water polar.
The polarity of water causes it to have a stronger intermolecular attraction than if it was nonpolar. (Imagine if you threw a bunch of magnets into a box). Since water sticks to itself so well (cohesion), it has a much higher melting point and it exists as a liquid at room temperature.
The polarity of water also makes it a good solvent (dissolver). Whenever an ionic compound like salt is introduced, the negative and positive poles of the water cause the salt to dissociate into its separate ions. Water is often called the ‘universal solvent’.