We want to investigate the collective dynamics of water at intermediate length scales (below the first structure factor peak and approaching the hydrodynamic range). To reconcile the existing experimental observations, we expect a maximum in the collective time in this region. Since in this Q-regime coherent and incoherent signals from D2O are contributing practically with the same intensity, it is mandatory to use polarisation analysis to disentangle both dynamic structure factors and provide new insight into this virgin territory.