This work analyses the water transport and ionic losses in bipolar membranes at water electrolysis cells conditions. In common bipolar setups, water is split at the bipolar interface between the anion exchange membrane (AEM) and the cation exchange membrane (CEM). Accordingly, ions (protons and hydroxide ions) are transported to the electrodes, carrying the water out of both membranes via electro-osmotic drag. These outfluxes plus the required water amount for the splitting process have to be compensated by water diffusion towards the bipolar interface. The effect of water transport on the polarisation behaviour is additionally shown. Mayerhöfer et al. [ACS Appl. Energy Mater., 3, 9635 (2020)] and Oener et al.[ACS Energy Lett., 6, 1 (2021)] decreased polarization losses and increased the current density range by reducing either the AEM or the CEM thickness, respectively. Our model validates these improvements by calculating the limiting current density caused by dehydration of the membranes. Further analysis shows that thinner AEM thicknesses decrease membrane voltage losses more than thinner CEM due to lower ionic conductivities and faster dehydration of AEMs. Thin CEMs on the other hand, are more efficient at increasing the limiting current density.