The ocean covers 70% of the surface of the planet, yet absorbs a remarkable 93% of the additional heat trapped by anthropogenic greenhouse gases due to its large size, the omnipresent stirring by wind, and the “high heat capacity” of water; yet the molecular basis for the high specific heat of water (CP) is not well known. It has long been established that specific heats are closely tied to molecular weight. Ironically, water has no fixed molecular weight: it exists as a bi-molecular fluid consisting of the singlet H2O form and an ensemble of hydrogen bonded forms in a temperature dependent equilibrium. We show that the mean molecular weight of water over the range 0-40°C is 82-79. The warming of water induces the breaking of hydrogen bonds (8.364 kJ/mol), increasing the population of the singlet H2O form at the expense of the hydrogen bonded forms. Although warming of sea water by 10°C yields only a 2% increase in free H2O, this accounts for some 36% of the energy consumed. Consequently, the high heat capacity of water, and water in sea water, is attributable (64%) to the large molecular weight of the hydrogen bonded forms, dominantly as the tetrahedral pentamer (H2O)5, and also (36%) to the energy required to break hydrogen bonds. The CP for pure water decreases with increased temperature due to the decrease in the ensemble molecular weight with warming, while the CP for sea water increases with increasing temperature due the work required to increase the translational energy of the large hydrated cations, which are dominantly Na(H2O)6 with a molecular weight of 131. Thus, it is the multiple forms of water and their hydrogen bonding that accounts for the high specific heat of both pure water and sea water.