A humidification–dehumidification (HDH) desalination system requires thermal energy to desalt seawater. An environmentally friendly approach to obtain thermal energy is to utilize solar energy using solar collectors. Either seawater or air (or both) are typically preheated by HDH desalination systems before these fluids are conveyed to the humidifier column. Compared with preheating only air or water, preheating both is preferred because improved performance and higher productivity are achieved. Many researchers have proposed dual preheated HDH systems utilizing two separate solar heaters/collectors for simultaneous air–seawater preheating. In this study, dual-fluid preheating is achieved using a single solar collector. The proposed simultaneous air–water solar heater (SAWSH) is a modified flat-plate collector designed for simultaneously preheating air and seawater before the fluids reach the humidifier. A thermodynamic study was conducted using formulated mathematical models based on energy and mass conservation principles. Then, the dual-fluid heating HDH system is compared with HDH systems in which only air or only water is heated. This work found that the former outperformed the latter. The daily and monthly performance levels of the system in terms of the outlet temperatures of air and water, distillate rate, and gain output ratio were calculated using the weather data of the hot and humid climate of Jeddah City, Saudi Arabia.