Did you actually try pedaling with the single fin? Should be easier than paddling... even with a single.
Running with one fin in any case, the boat will rock back and forth, but you can quickly adapt to the motion and still be able to use the Drive to get home. This next picture shows the GPS track of an Adventure running with one fin at "fast cruise". Obviously a little slower with less thrust available, it nevertheless propels the boat quite adequately when properly tuned and adjusted (especially with a Turbofin).