Stone County Sheriff Doug Rader said five people remain missing and seven others were taken to hospital after a Ride the Ducks boat sank on Table Rock Lake in Branson.
A spokeswoman for Cox Medical Centre said four adults and three children arrived at the hospital shortly after the incident. Two adults were in critical condition and the others were treated for minor injuries, Brandei Clifton said.
Mr Rader said stormy weather is believed to have caused the capsizing. Another duck boat made it safely back to shore.
Steve Lindenberg, a National Weather Service meteorologist in Springfield, Missouri, said the agency issued a severe thunderstorm warning for the Branson area on Thursday evening, and winds reached speeds of more than 60mph.
“It’s a warning telling people to take shelter,” he said.
Mr Rader said an off-duty police officer working security for the boat company helped rescue people after the accident.
Dive teams from a number of law enforcement agencies were assisting in the effort, but the sheriff said the divers had ended their search for the night.
The National Transportation Safety Board said on Twitter that investigators will arrive on Friday morning.
Suzanne Smagala, of Ripley Entertainment, which owns Ride the Ducks in Branson, said the company was assisting authorities with the rescue effort. She added this was the Branson tour’s first accident in more than 40 years of operation.
Branson is about 200 miles south east of Kansas City and is a popular vacation spot for families and other tourists looking for entertainment ranging from theme parks to live music.
Duck boats, known for their ability to travel on land and in water, have been involved in other deadly incidents in the past. They include one in 2015 in Seattle in which five college students were killed when a boat collided with a bus, and one in 1999 that left 13 dead after the boat sank near Hot Springs, Arkansas.
Safety advocates have sought improvements to the boats since the Arkansas incident. Critics argued that part of the problem is numerous agencies regulate the boats with varying safety requirements.
Duck boats were originally used by the US military in the Second World War to transport troops and supplies, and were later modified for use as sightseeing vehicles.