About 32 people were also injured after the boulder fell on the boats and 20 people are still missing, Brazilian state media Agência Brasil reported.
Nine people were taken to nearby hospitals while authorities investigate the incident.
According to Romeu Zema, the governor of Minas Gerais, a landlocked state in southeastern Brazil, the rock has been loosened at Lake Furnas in Capitólio as a result of heavy rainfall.
“I am in solidarity with the families at this difficult time,” Zema wrote on Twitter. “We will continue to act to provide the necessary protection and support.”
A video posted to social media showed tourists on other boats in Furnas Lake shouting “get out” just before the rock fell.
Colonel Edgard Estevo, commander of the Minas Gerais fire brigade, said at a press conference that the search for the missing persons would continue, but the divers would suspend their search overnight for their safety.
Brazilian President Jair Bolsonaro tweeted the video, adding that the navy has deployed an auxiliary force to join the search and rescue.