<p>Solar-driven interfacial water evaporation offers a promising and sustainable solution for freshwater production by harnessing abundant solar energy. Recent research has focused on enhancing this process using selective solar absorbers embedded with photothermal nanomaterials. This study investigates the thermo-fluid performance of a novel seawater desalination system featuring a copper sulphide (CuS) nanomaterial-infused absorber embedded in a Darcy–Forchheimer porous matrix. The system is modelled using nonlinear partial differential equations based on mass, momentum, and energy conservation, which are transformed into coupled ordinary differential equations via similarity variables. The resulting boundary value problem is solved numerically using the shooting method with a Runge–Kutta–Fehlberg algorithm. Simulation results reveal that increasing the CuS nanoparticle volume fraction (<i>ϕ</i>) enhances the thermal conductivity of the absorber, leading to increased steam velocities, thinner thermal boundary layers, and significantly reduced skin friction. The interfacial Biot number at the absorber–seawater interface (<i>Bi</i>_<i>f</i>) positively influences heat flux and the Nusselt number by promoting more effective convective heat transfer into the seawater. Similarly, an increase in the Biot number at the steam–condenser interface (<i>Bi</i>_<i>s</i>) enhances condensation efficiency and draws steam more rapidly from the evaporation surface, further boosting steam velocity. Conversely, increasing the slip parameter (<i>S</i>) and the steam Eckert number (<i>Ec</i>) reduces the Nusselt number due to diminished temperature gradients at the absorber surface. These findings provide key physical insights into absorber–fluid interactions and highlight the potential of CuS-based photothermal absorbers to significantly improve the thermal efficiency and freshwater output of decentralised solar desalination systems.</p>