First AirboatThe first airboat, called the Ugly Duckling, was built in 1905 in  Nova Scotia, Canada by a team led by Dr. Alexander Graham Bell.
It was used to test various engines and prop configurations. An associate of Dr. Bell, Glenn Curtiss (of airplane manufacturing fame)
is reported to have registered the first airboat in Florida, USA in 1920.
It was called the Curtis Scooter and it had a closed cockpit design.

A story in the Nov/Dec 2009 issue of Airboating Magazine that showed an airboat called a Free Bottom Craft that was built in the mid to late 1920s, using a wood hull, a Curtiss aircraft engine and was on exhibit at a New York boat show. The airboat was built by Charles Post and Herbert Ballantine in Huntington, NY and tested on the Long Island’s Hewlett Bay.

By the 1930s homemade airboats began appearing in the swamps and marshes of Florida and Louisiana. One company in Florida claims to have been providing airboat rides as entertainment since the mid 1930s. Over the years a variety of designs were tried and through trial-and-error, the standard design used today arose: an open, flat bottom boat with an engine mounted on the back, the driver sitting in an elevated position, and a cage to protect the propeller from objects flying into them. One well documented case of a homemade design (though not the first) was an airboat built by staff at the Bear River Bird Refugee near Brigham City, Utah in the 1940s. It appears to have involved collaborative efforts by three employees of the refuge – Leo Young, G. Hortin Jensen and Cecil Williams.

A story in Ducks Unlimited magazine in 1987 mentioned Young and Jensen and dated the building of the first boat in 1950. Refuge records, however, show the first boat came into use in 1943, with several photos of running air boats dated 1947. Prior to the introduction of the airboat, refuge biologists had to either walk through shallow water and deep, sticky mud or push flat-bottom boats with long poles. Staff had experimented with a boat called the “Mud Queen,” which had small paddle wheels on either side that pushed the boat. They built their first airboat nicknamed “Alligator I” from a flat-bottom boat pushed along by an aircraft engine purchased for $99.50. Young reported that he called the first airboat an “air-thrust boat.” Once word got out about the boat, Leo Young built and sold boats all over the world.
(http://creativecommons.org/licenses/by-sa/3.0/)