About 32 people were also injured after the boulder fell on the boats and 20 people remain missing, Brazilian state media Agência Brasil reported.
Nine people were taken to nearby hospitals while authorities investigate the incident.
The rock was loosened at Furnas Lake in Capitólio due to heavy rains, according to Romeu Zema, the governor of Minas Gerais, a landlocked state in southeastern Brazil.
“I stand 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 that was posted on social media showed tourists on other boats in the Furnas Lake shouting “get out of there” right before the rock fell.
Col. Edgard Estevo, commander of the Minas Gerais Fire Department, said during a press conference that the searches for the missing persons would continue, but the divers would stop their search during the night for their safety.
Brazilian President Jair Bolsonaro tweeted the video, adding that the Navy deployed a relief force team to join the search and rescue efforts.
