By 7:30pm, winds of 102 km/h had been reported.
Stone County Sheriff Doug Rader said early Friday morning that 14 people survived the incident.
Life jackets were on board the boat, Rader said. Rader said crews will stay on the scene all night.
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In a statement Friday, Boston Duck Tours said "We continuously monitor the weather conditions from our three locations".
He said it is unknown whether the captain or Ride the Ducks staff checked the forecast prior to going on the lake.
"Very sad to hear about this awful accident - prayers for all those involved and the first responders who are assisting", Missouri Gov. Mike Parson said on Twitter . "We are thankful for the fearless first responders and dive crews, whose quick and decisive actions have saved many lives, and we continue to pray for their safety as their search, rescue, and recovery operations continue". Parson also flags in the state to be flown at half-staff for a week. The National Transportation Safety Board said the tug's mate was distracted by cell phone calls and a family emergency. People left flowers and roses in the parking lot and on cars left in the departure area for this Branson attraction in memorial for those who lost their lives. Onlooker Jennie P. Carr, who was on the Branson Belle Showboat as people had dinner, recorded footage of the boat's nose sinking below the water. Seven people were taken to the hospital; two of the wounded are in critical condition, according to a local hospital.
Steve Lindenberg, a meteorologist in Springfield, Missouri some 45 miles (72 kilometers) north of Branson, told AFP a string of severe thunderstorms had barreled through the area causing significant tree damage and downed power lines.
The boat capsized Thursday evening when a thunderstorm hit the area and brought winds that approached near-hurricane speeds.
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The man-made lake where the boat sank is a popular tourist draw located in southern Missouri near the city of Branson, on the border with Arkansas.
Among the rescuers was an off-duty sheriff's deputy on the Branson Belle who "jumped in and helped", Rader said.
The Missouri Highway Patrol Dive Team was set to take over the search Friday morning from a county dive team that scoured the waters Thursday night, officials said.
President Donald Trump tweeted his condolences Friday, extending his deepest sympathies to the families and friends of those involved.
Suzanne Smagala, of Ripley Entertainment, which owns Ride the Ducks in Branson, said the company was assisting authorities with the rescue effort.
The company Ripley Entertainment owns the capsized boat, having added Branson's "Ride the Ducks" attraction to its roster previous year.
Branson, a popular family vacation destination, is about 200 miles southeast of Kansas City, Missouri.
Land-to-water boats are a hit tourist attraction for sightseers around the U.S. - but the amphibious vehicles have a long history of deadly accidents stemming back to the 1990s.
In a phone interview with "CBS This Morning", the president of the company that owned the duck boat said that "no one was expecting" the storm but the boat "shouldn't have been in the water if what happened happened".