The expansion of coastal infrastructures is increasingly recognised as a major driver of marine biodiversity loss. Urban and industrial ports are particularly human-impacted marine environments, where submerged artificial surfaces are often structurally less complex and colonised by non-indigenous species. Eco-engineering approaches that increase habitat complexity are emerging as Nature-based Solutions (NbS) to enhance biodiversity. This study aims to assess the effects of ad hoc Biodiversity Support Units (BSUs) for enhancing biodiversity associated with submerged quay walls in the Port of Genoa. To achieve this goal, large BSUs (L-BSUs; 3D printed in concrete with two alveolation types; dimensions: 100x100x20 cm) were deployed at two sites, at depths of 0.5 m, 3 m, and 5.5 m, and compared to the flat walls. L-BSUs were monitored every three months over 15 months using ROV-based imagery to assess the temporal dynamics of benthic assemblage composition, taxon richness, and percent cover across sites, depths and substrate types. Small 3D printed BSUs (S-BSUs), deployed at 0.5 m depth, were also periodically retrieved to allow a finer in vivo taxonomic identification of small-sized organisms. Multivariate analyses and GLMMs showed a clear temporal trajectory: early-stage assemblages were dominated by turf and colonial ascidians, progressively replaced by serpulid polychaetes (mostly Hydroides spp), and bryozoans, and later by filamentous and encrusting algae. At 0.5 m depth, both alveolation types supported about 50% more taxa than adjacent flat wall, while the effects on total cover were less pronounced. Overall, L-BSUs consistently enhanced benthic biodiversity and promoted more complex assemblages over time, indicating their potential as NbS in the ecological rehabilitation of urban marine environments.