If you take the ferry with a car, you will have AFIP checking your car documents, VTV, RTO, and so on, as well as your driving license (so don't be more than 6 months in-country if you only have a foreign DL) and your "cedula verde" for the car.
I expect to do the run to Montevideo next weekend so I can report back. Fortunately all my documentation is now in order, I think.