The victims, who were touring an underground river known as the Neglinka, are believed to have become trapped by rising water levels following a downpour and were unable to escape to the surface.
Among those who drowned were the tour guide and seven members of his party, which included a minor, local media said.
"The bodies of all participants of the excursion have been found," investigators said, adding formal identification of the victims was underway.
Moscow Mayor Sergei Sobyanin called the incident a "terrible tragedy" on social media and expressed his condolences.
Several tour operators offer trips into the vast tunnels of the capital's sewer system, some of which were constructed during the 19th century.
The legality of such trips is disputed, and investigators said Monday they had launched a criminal case into an "illegal excursion".
One of the tour organisers has already been detained, Russia's Investigative Committee said Tuesday, adding that another defendant is thought to have fled to the United Arab Emirates.
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