The present study proposed the green synthesis of silver (Ag) nanoparticles using aqueous extract of Acetabularia acetabulum followed by their fabrication onto montmorillonite (MMT). Fourier transform infrared (FTIR) spectra revealed the involvement of multiple functional groups in the reduction of silver ions to Ag nanoparticles and their stabilization on MMT. The obtained MMT-Ag nanocomposites were characterized by UV–visible spectroscopy, powder X-ray diffraction (XRD), particle size analysis (PSA), scanning electron microscopy (SEM) and Energy Dispersive X-Ray (EDX) analysis. The synthesised Ag nanostructures were found to be cubic shaped with average size ranges from 37nm to 60 nm. The seaweed mediated MMT-Ag nanocomposites were evaluated for their potential antimicrobial properties against the isolated biofouling bacteria. Maximum bactericidal activity was recorded against S. aureus followed by E. coli, M. flavus, Pseudomonas aeruginosa, B. cereus, M. leteus and B. subtilis. In addition, the viability of incorporating MMT-Ag nanocomposites in paint was examined where a significant inhibition of marine fouling bacteria was exhibited by the panel coated by MMT-Ag nanocomposites-based paint as compared to water-based paint. The addition of MMT-Ag nanocomposites in water-based paint was also found to be effective against corrosion from marine water. The present study shows cytotoxicity of MMT-Ag nanocomposites as nanoclay/metallic nanocomposites against A. salina with LD50 values of 200±3.4 µg/ml. The results of the present study suggested the application of A. acetabulum extract as a good bio-resource for the synthesis of Ag nanoparticles and their implementation to combat marine biofouling on ship hulls.