Moving to water deep enough to get the dog’s feet off the ground is a sound move. Much of a dog’s advantage in a fight is speed and mobility. A swimming dog is extremely vulnerable in comparison to one on it’s feet … even the ones designed for swimming like labs. This is one of the few situations that I’d probably pull a knife over a pistol if I had both … mainly for the "legal hassles" … no doubt that the dog was attacking because of the range involved, didn’t want to kill it and the knife was a last resort, etc. … and all that rot.
The pistol would be more for the dog owner ... if he turned belligerent.
Depending on what you’re fishing with, the pole and bait can be an optional non-lethal control rod for a swimming dog. A fly rod and some baits would be useless, but some rigs can be effective. Take it from someone that snagged the snout of a swimming dog with a Lucky 13 (three treble hooks) on a 6 foot fiberglass rod with about 6 inches of 27 lb. test spiderwire let out… steer him wherever you want, just keep the rod bowed a little … and don’t let him come straight down the rod at you (could break the rod). The owner was a bit upset till I told him the vet bill would be a lot cheaper than what I’d sue him for if the dog had gotten to me. The dude also got a funny look when I asked him which vet he was taking the dog to so we could call ahead and have the guy expecting the dog … and so I’d know where to pick up my favorite fishing plug. (also told the vet to feel free to cut any hooks he needed to, but not the body itself).
It may be a little bit "hick" to be concerned about the plug, but hey! … We’re talking a glass-eyed 7/8 oz. Hand-painted sunflower-pattern Lucky 13 here ! … and it wasn’t my dog.
(edit to correct weight of plug)