This is a WWII GMC model DUCW, a 2.5-ton 6×6 amphibious vehicle that used six wheels on land and a propeller when in the water. Essentially sea-going trucks, they were crucial for ferrying supplies from ship to shore. The vehicles were naturally nicknamed Ducks, and Canadian war correspondent Dick Sanborn reported this incident in 1943:
During the invasion of Italy, when hundreds of ducks plied their way back and forth carrying anti-tank guns, mortars and ammunition across the Messina straits, a British destroyer raced cockily past one group. From the bridge twinkled a signal lamp in Morse. Deeply offended, the officer in charge of the ducks translated the message: “Quack, quack.”
Sources:
Sanborn, Dick. “Amphibian Wins Spurs in New Canadian Push.” The Winnipeg Tribune, 11 December 1943, p. 1.