This White Paper describes the scope of the Centre for Molecular Water Science (CMWS), a collaborative, cross-disciplinary research initiative that shall use a highly dedicated laboratory infrastructure, the most advanced experimental techniques, leading theory/simulation tools and forefront photon facilities to answer the most important questions in molecular water science. The key scientific challenges, preliminary work, objectives, methodologies and infrastructural needs for five major scientific research areas (pillars) have been identified during the past three years by a group of more than 140 scientists from more than 15 countries. The results are compiled and summarised in this document. This task would not have been possible without the continued support from the host laboratory DESY in Hamburg (Germany). Special thanks to the Chairman of the DESY Board of Directors, Prof Helmut Dosch, and the Director in charge of Photon Science, Prof. Edgar Weckert.