Water is known to exhibit fascinating physical properties at high pressure and temperature. Its remarkable structural and phase complexities suggest the possibility of exotic chemical reactivity under extreme conditions, although this remains largely unstudied. Detonations of high explosives containing oxygen and hydrogen produce water at thousands of kelvin and tens of gigapascals, similar to conditions in the interiors of giant planets. These systems thus provide a unique means of elucidating the chemistry of 'extreme water'. Here, we show that water has an unexpected role in catalysing complex explosive reactions--contrary to the current view that it is simply a stable detonation product. Using first-principles atomistic simulations of the detonation of the high explosive pentaerythritol tetranitrate, we discovered that H(2)O (source), H (reducer) and OH (oxidizer) act as a dynamic team that transports oxygen between reaction centres. Our finding suggests that water may catalyse reactions in other explosives and in planetary interiors.