Water is a valuable reserve that shall be efficiently treated for sustainable growth and development. Metal nanoparticles (MNPs) and graphitic carbon nitride (gCN) have received immense attention for several applications. In this paper, the facile synthesis of MNPs/gCN hybrid photocatalysts based on sonodispersion, and in-situ reduction and photodeposition is described. Photodeposition of two different metals, namely gold nanoparticles (Au NPs) and silver nanoparticles (Ag NPs) with different loading ratios was studied. The morphological, structural, electronic, optical, and surface properties of the prepared catalysts were investigated. The photocatalytic activity of the prepared catalysts was evaluated for photooxidation of organic water pollutants under visible light irradiation using methyl orange (MO) and rose bengal (RB) as model dyes. The photo- and recycling stability, and the photoelectrochemical properties of a selected photocatalyst were also studied. The results confirmed the evident formation and characteristics of the prepared MNPs/gCN materials. The catalysis measurements revealed the promoting effect of the Au and Ag NPs toward the adsorptive removal and photooxidation of MO and RB. Overall, the Ag NPs demonstrated higher catalytic activity for dye oxidation compared to Au NPs. The Au/gCN resulted in adsorptive removal of 3.87-11.80 % of MO, whereas the Ag/gCN led to adsorptive removal of 66.45-85.33% of MO within 1 h in dark. Also, Ag/gCN catalysts exhibited an extraordinary adsorption ability toward RB that is six-time higher than that of Au/gCN. Interestingly, the Ag/gCN photocatalysts demonstrated the highest photocatalytic activity with 100% photooxidation of MO, and RB within 20 min, and 15 min under visible light irradiation, respectively. The high photocatalytic activity could arise from the coupled plasmon absorption, enhanced visible light harvesting, and efficient charge separation. Besides superior photocatalytic activity, the Ag/gCN catalyst expressed promising photostability, reusability, and photoelectrochemical responses. The obtained results indicate that MNPs/gCN materials could serve as efficient visible-light-sensitive catalysts for photocatalytic water treatment.