This paper describes, from a thermodynamic point of view, the physicochemical conditions, under which water behaves as an explosive. This phenomenon occurs frequently in hydrothermal and volcanic systems when water is brutally shifted from its initial equilibrium state. Water (either liquid or gas) becomes metastable or unstable and reequilibrates by violent demixing of a liquid-gas mixture. In a first step, a phenomenological approach of metastability is given in a one-component perspective, introducing the notion of spinodals and delimiting the extent of metastable fields. The physical mechanisms (bubble nucleation, cavitation, spinodal decomposition), which are involved in these explosive transformations of water, are detailed in what relates to the natural eruption topic. The specific thermodynamic properties (P-v-T-H-U) of metastable water are presented by using the reference Wagner and Pruss equation of state. Then, the mechanical work produced by the different possible physical transformations, including decompression, vaporization, isobaric heating, and exsolution, involved in water explosions are quantified. The classic calculation of the energy balance under the reversible assumption is extended, proposing a pathway to take irreversibility into account. This model can be used to estimate magnitude of volcanic impacts from scaling laws based on explosion energies.