<p>Citizen science has become a powerful tool for generating extensive ecological data and fostering public engagement in environmental monitoring and conservation initiatives. In marine environments, where monitoring is challenging due to logistical demands and financial constraints, engagement of recreational divers in citizen science projects offers a promising avenue for expanding data collection efforts over large spatial and temporal scales. Here, we evaluate the accuracy of ecological data collected by 118 volunteer divers in three countries in the Mediterranean region by comparing their observations with those of expert scientists during 2012–2016. Using standardized monitoring protocols across multiple sites, we assessed volunteers’ abilities to identify marine species, threats and impacts, estimate abundances, and correctly report species’ absence. Our results demonstrated a notable success rate (exceeding 70% in many cases) in the detection and assessment of multiple species and threats. Nonetheless, volunteer divers frequently misidentified sessile taxa and fish species and often failed to detect less conspicuous environmental stressors, such as tissue necrosis indicative of mass mortality events. Diving experience and site familiarity were found to significantly influence the quality of information reported by non-experts, with experienced and site-acquainted divers demonstrating higher data accuracy. While citizen science can greatly support ecological research and conservation, our findings indicate that comprehensive training and stringent data-validation procedures are indispensable for reliably capturing complex or less conspicuous ecological variables.</p>