Single ionization from water molecules by impact of protons, alpha particles and C6+ ions is studied. The post- and prior-versions of the continuum distorted wave-eikonal initial state (CDW-EIS) model within an independent electron approximation are employed to compute double differential cross sections. To avoid the complexity of using numerical molecular continuum states in the cross-section calculations, effective Coulombic continuum wavefunctions are employed. However, this may lead to the appearance of post-prior discrepancies and this fact is examined in detail. Moreover, the influence of the dynamic screening on this behaviour is studied. In addition, the contribution of different molecular orbitals to the angular spectrum is analysed for several ejection electron energies. Finally, the sensitivity of CDW-EIS calculations to the representation of the initial bound molecular orbitals is investigated.