  We simulate liquid water between hydrophobic walls separated by 0.5 nm, to
study how the diffusion constant D_\parallel parallel to the walls depends on
the microscopic structure of water. At low temperature T, water diffusion can
be associated with the number of defects in the hydrogen bonds network.
However, the number of defects solely does not account for the peculiar
diffusion of water, with maxima and minima along isotherms. Here, we calculate
a relation that quantitatively reproduces the behavior of D_\parallel, focusing
on the high-T regime. We clarify how the interplay between breaking of hydrogen
bonds and cooperative rearranging regions of 1-nm size gives rise to the
diffusion extrema, possibly relevant for both bulk and nanoconfined water.
