Due to the need for fresh water and desalination systems, especially those powered by renewable resources, solar still systems represent a suitable technology. Therefore, this study explores a cost-effective improvement of hemispherical solar stills by incorporating floating plastic tubes coated with copper oxide (CuO) nanoparticles to improve thermal performance and freshwater productivity. The proposed modification aims to increase solar energy absorption, improve heat distribution, and reduce the surface tension of water, which accelerates the evaporation process. Four identical hemispherical solar stills were tested under identical environmental conditions in El Ouad, Algeria. A standard hemispherical solar still (SHSS) and three modified designs: HSS&3/4FPP, HSS&1/2FPP, and HSS&1/4FPP, where the tubes floated at 3/4, 1/2, and 1/4 of their height above the basin water surface, respectively. The results showed that all modified designs improved thermal performance compared to high-efficiency solar stills (SHSS), with the HSS&1/4FPP system achieving the best performance, achieving a 63.26% increase in freshwater production, a 65.22% increase in thermal efficiency, and a 149.74% increase in exergy efficiency. This configuration also reduced the cost of freshwater production by 50.13%, demonstrating the impact of increased tube immersion on enhancing heat transfer to the water and reducing surface tension. The results highlight the effectiveness of using CuO-enhanced floating elements in improving the efficiency and economic feasibility of solar still systems, offering a practical solution for freshwater generation in remote or arid regions.