The DUKW wasn't designed as a combat vehicle, it's purely a transport. In fact, it's the same chassis and drivetrain as the standard GMC CCKW (Deuce-n-a-half).
Although it's unarmored and still relatively slow in water, it's primary advantage in-game would be the ability to get troops or supplies to a base as fast as a Jeep could. Couple a few DUKWs with some M8s and Jeeps, and you've got a high-speed convoy capable of covering a pretty decent stretch of land in a relatively short time.
I think a DUKW and a CCKW would be excellent additions to the game. They're unarmored and unarmed, so they don't pose an offensive threat to anyone or anything, much like a C-47, but they do add a speed element to the game that at present, is only available in the C-47.
Again, just my opinion...
Jeff
