The Ministry of Public Health has a website with maps showing the polluted and unpolluted beaches within the Campania region: http://www.arpacampania.it/baln2009web/MappaBalneazioneLitoraleFlegreo.pdf
The blue dots are clean areas and red dots are polluted beaches.
Another word of caution. If the beaches are polluted, more than likely the fish and shellfish are also polluted. If you are currently pregnant, try to shy away from the fruits of the sea. For everyone else, ask your local market from which area the fish come.