I did the ferry boat ride from BA to/from Colonia as a US citizen.

Very easy and painless, and I had no visa.

The only hassle was having to reserve the boat ride at one window, then having to do the ticketing at another window, then having to pay at still another window, then picking up the ticket at the second window. And speaking minimal Spanish was of no help. But the friendly staff was cool.

The back-to-back immigrations counters are a hoot though.