The year-long operation came to a head on Tuesday after police recovered 31 Roman vases and other items, Il Secolo XIX reported.
It is believed the gang then sold on the relics, as a flyer advertising "unique gifts for lawyer, doctors and politicians", was also found.
The gang was active along the Ligurian coast, particularly the waters off Imperia and Albenga. The criminal network included a member of staff at Imperia council and a number of professional divers, the newspaper said.
The treasures uncovered included a Roman centurion's dagger and a Fiat bomber shot down during the Second World War.