Duck boats are a U.S. tourist attraction that have unfortunately seen several high-profile accidents (the most recent killed 17 people on Table Rock Lake in Branson, Missouri). Yet before they were a controversial tour vehicle, the Allied troops during World War II used them to get artillery, supplies and even soldiers ashore.

DUKW boats made their first appearance in combat during the 1943 invasion of Sicily known as Operation Husky. The name “DUKW” corresponded to General Motors’ manufacturing code (“D” is “1942 model,” “U” is “amphibious,” “K” is “all-wheel drive,” and “W” is “dual rear wheels”). Soldiers simplified this by calling them “ducks.”

Like actual ducks, these boat-trucks with wheels looked a little awkward. But their weird design is what allowed troops to drive the vehicles from water onto land—a crucially important task, according to Joseph Balkoski, historian of the Maryland National Guard and author of several books on military history.

“Everything that the United States did in World War II was going to be based on invasions against enemy-held coastlines,” he says. “So we had to get man and equipment from the water to the shore in as expeditious a manner as possible.”

Before, large ships had to sail up to the shore for the time-consuming process of unloading. Duck boats simplified this because they were pre-loaded vehicles that drove right out of a ship, across a few miles of water and onto the sand.

“That was obviously revolutionary because up until that time you had to unload a vessel when it came to shore or a dock and that was a very, very time consuming operation,” he says.

In the U.S. military, duck boats were Army vehicles that truck drivers trained to use. As land trucks, they were pretty useful, especially with their added ability to transition from land to river travel. In March 1945, the Allies used duck boats to cross the Rhine River in Germany.

Overall, Balkoski says that the boats were extremely successful in both Europe and the Pacific. However, there were some hiccups during the D-Day invasion of Omaha Beach in Normandy, France.

“What happened on Omaha Beach in particular was that we tried to do too much with ducks,” Balkoski says. In addition to loading the boats with food, medicine, ammunition and other supplies, officials also loaded a heavy howitzer or cannon onto each of 12 boats.

“That normally would not have been an issue in calm waters,” he says. “A duck is designed to carry a heavy load… But D-Day was an operation that was carried out in very imperfect weather. There were three-to-five-foot waves off shore of Omaha Beach and that’s very unusual, it’s normally much calmer than that.”

After rolling off of the mothership, the duck boats carrying howitzers began to sink almost immediately. Only one of those ships survived with its cannon. The rest of the howitzers and supplies were lost to the water, and, tragically, so were some soldiers.

“When used properly they were an incredibly brilliant invention, really one of those great Allied inventions that helped win the war,” Balkoski says. “But when stretched to the limit they had this grievous weakness.”

The U.S. produced a lot of military vehicles during the war, including about 20,000 DUKW boats. When the fighting ended, the military no longer needed all of its WWII vehicles, so the solution was to either destroy them or find a civilian use for them. Starting as early as 1946, entrepreneurs began using former Army duck boats to give water tours.

Today, some of the duck boats you can ride during a tour are refurbished models from WWII or the Korean War. Others, like the one that capsized in Branson, are replicas. But as a whole, are they safe for civilian use?

“It’s very hard to make a truck that goes on the water and then drives on land, and the Achilles’ heel was that even in remotely rough waters they were very, very unsafe,” Balkoski says. In that regard, he says that he wasn’t extremely surprised to hear of the fatal accident in Branson. Still, he doesn’t think that all tourist attractions should stop using duck boats.

“I have taken my own kids on duck boats several times and never gave it a thought,” he says, but, he adds, he would only advise riding a duck boat in calm weather. As the Allies learned on D-Day, rough water can render them unstable—and unsafe.