I haven't yet had any unpleasant dog encounters on a river (other then the rare occassion that Cordog outfishes me) but if I ever felt threatened I would definitely have my knife handy and wouldn't hesitate to use it, I don't care if its the owners fault that the dog is aggressive, I'm going to defend myself.
Only ? is, I have a Kershaw 3 blade knife, do I filet the dog or gut him?