Funny you say that. I think the originals were Higgins. And that is exactly what they were. Duce n half truck equivalent of that time that had a tub put on them to float.We were there last week. Those ducks look like a duck boat but are just a land vehicle with no capability to go in water.
Edit: Looking at your link, they do go to water so I'm corrected. We drove right next to one and it appeared to have nothing but a truck chassis with a fake fiberglass bow on it.
No means of propulsion in the rear, but maybe there is something that is retractable? Don't know.
There should be a propeller that I think is recessed into the center of the body.