The effect of water addition during preparation of a CH₃NH₃PbI₃ layer on the photodynamics is studied by femtosecond transient absorption. Both the regular perovskite and the aqueous analogue show charge thermalisation on a timescale of about 500 fs. This process is, however, less pronounced in the latter layer. The spectral feature associated with hot charges does not fully decay on this timescale, but also shows a long-lived (sub-ns) component. As water molecules may interfere with the hydrogen bonding between the CH₃NH₃ ⁺ cations and the inorganic cage, this effect is possibly caused by immobilisation of cation motion, suggesting a key role of CH₃NH₃ ⁺ dipole reorientation in charge thermalisation. This effect shows the possibility of controlling hot charge carrier cooling to overcome the Shockley–Queisser limit.