Institutes often lack funds and manpower to perform large-scale biodiversity monitoring. Citizens can be involved, contributing to the collection of data, thus decreasing costs. Underwater research requires specialist skills and SCUBA certification, and it can be difficult to involve volunteers. The aim of this study was to involve large numbers of recreational divers in marine biodiversity monitoring for increasing the environmental education of the public and collecting data on the status of marine biodiversity. Here we show that thousands of recreational divers can be enrolled in a short time. Using specially formulated questionnaires, nonspecialist volunteers reported the presence of 61 marine taxa encountered during recreational dives, performed as regular sport dives. Validation trials were carried out to assess the accuracy and consistency of volunteer-recorded data, and these were compared to reference data collected by an experienced researcher. In the majority of trials (76%) volunteers performed with an accuracy and consistency of 50-80%, comparable to the performance of conservation volunteer divers on precise transects in other projects. The recruitment of recreational divers involved the main diving and tour operators in Italy, a popular scientific magazine, and mass media. During the four-year study, 3825 divers completed 18757 questionnaires, corresponding to 13539 diving hours. The volunteer-sightings-based index showed that in the monitored area the biodiversity status did not change significantly within the project time scale, but there was a significant negative correlation with latitude, suggesting improved quality in the southernmost areas. This trend could be related to the presence of stressors in the northern areas and has been supported by investigations performed by the Italian Ministry of the Environment. The greatest limitation with using volunteers to collect data was the uneven spatial distribution of samples. The benefits were the considerable amounts of data collected over short time periods and at low costs. The successful development of citizen-based monitoring programs requires open-mindedness in the academic community; advantages of citizen involvement in research are not only adding large data sets to the ecological knowledge base but also aiding in the environmental education of the public.