We investigate how the treatment of electrostatic interactions influences the magnitude of the thermal polarization of water. We performed non-equilibrium molecular dynamics simulations of the extended simple point charge model of water under a thermal gradient, using two different systems: a water droplet confined in a spherical wall where the interactions are computed exactly using the Coulombic potential and a periodic prismatic box using the Wolf and 3D Ewald methods. All the methods reproduce the thermal polarization (TP) of water as well as the direction of the TP field, but the standard implementation of the Wolf method overestimates the strength of the TP field by one order of magnitude, showing that this method might be problematic in simulations involving temperature and/or density gradients.