Not quite an Amphicar, but it has wheels for driving on land and it can float and drive in the water... http://www.beamng.com/threads/huge-update-2-1-amphibious-armored-vehicle-btr-80-БТР-80-for-beamng-0-7.14971/ But I agree, an actual Amphicar would be cool, and I thought about such an idea while working on this mod. Too bad it would take so much work.
The bkr-80 or something like that is an amphibious vehicle, that might be the closest you'll get. EDIT: ninja'd