Metal-doping is a technique used to modify the surface, crystalline, textural, and/or optical properties of the photocatalytic semiconductors, giving greater photocatalytic degradation performances. In this study, a MgO semiconductor catalyst material was doped using copper (Cu) (3% wt) and zinc (Zn) (3% wt), respectively, via a simple co-precipitation synthesis technique. The synthesized 3% Cu-MgO and 3% Zn-MgO were then tested for the photocatalytic degradation of methylene blue dye (MB) as a model pollutant under neutral pH for 120?min. The catalyst dosage used was 500?ppm and the initial MB concentration was maintained at 11?ppm in both cases. After 120?min, the 3% Zn-MgO revealed a 68.85?±?2.31% MB degradation efficiency, whereas the 3% Cu-MgO revealed a 63.91?±?2.17% MB removal efficiency. Moreover, the synthesized pure MgO material was also employed for the photocatalytic degradation of MB under the same conditions (as a control), revealing only a 41.12?±?1.34% MB removal efficiency. These MB removal efficiencies suggest the importance of metal-doping in improving the photocatalytic degradation process. Moreover, the unit treatment cost of MB solution using the proposed photocatalytic process reached $7.95/m3. This study directly addressed UN SDG 6 “Clean Water and Sanitation” and 14 “Life below water” through the conservation of the marine environment by effectively treating wastewater prior to its exposure to the marine environment.