THe amphibious cars in KotCS are neat, but I don't think they're "Ducks", as most know them, from World War 2. In Pittsburgh there is a tour you can take that goes on land and river by actually riding in vintage World War 2 Ducks restored and used as tour buses basically.
It literally drives from downtown streets right into the Monongehela River. Neat thing to do in Pittsburgh I suppose but I've never done it myself. I'd like to but don't have time it seems.
Here is a converted Duck for touring London:
As you can see, it's REALLY huge by comparison to the amphibious car in KotCS, at least by my recollection of the vehicle. I could be off as I don't have it here to freeze-frame, but it really wasn't much larger than the Jeep-like car that the Erena/Irena (whatever) and her Russian Driver were in beside the amphibious car.
Here's a World War 2 Duck converted by Russia, probably as part of the lend-lease program. Russia got lots of Ford, Chevy, and other U.S. manufacturerd vehicles for use in their war effort.
As you can see, there's no way this is the "Duck" we see in KotCS, even though this is a Soviet converted Duck. The U.S. actually didn't appreciate the amphibious vehicles like the Russians did. Russians faced a greater obstacle in rivers than the western allies faced. Rivers were a problem for the U.S./British/Canadians of course, but the Russians truly faced a myriad of water obstacles from large rivers to small streams, on an almost daily basis of their offensives, so they actually utilized amphibious vehicles extensively, and improved on existing U.S. designs, to suit their needs.
This is a German Schwimmwagen, or essentially an amphibious version of the Kubelwagen which was a mainstay vehicle used by the Germans.
It's much more the size of the amphibious vehicles in KotCS, I think anyway from what I remember of them anyway, but they're not the same shape, and not exactly the same size.
Ultimately, on the Soviet vehicles in the movie, there's a 50/50 chance if they really exist. It's not a design I immediately recognized, but it's definitely smaller than a "Duck" (which I did hear Jones call it that too). Russians had a few amphibious vehicles of various sizes, some they made based on US designs and others I think they whipped up themselves. The vehicle in the movie, if it's real, likely was leftover WW2 stock though. I'd say the same about the cargo trucks and the "4WD" vehicle that Irena was in at the start of the chase.
The tree chopper thing was 100% ficitonal, though very neat I think, and it looked believable the way it was designed.