I think that it is probably true that all dogs will instinctively try to swim; just like any person will flounder around and try to swim if they are thrown into the water. How well they are actually able to swim is where the critical issue is at. Just as some people are more bouyant in the water than other people; so are some dogs able to swim easier than others because of either their body build or because of their coat length.
Any dog that has a long, thick coat is going to have trouble swimming because the coat will weight them down when it gets wet, just as we would, if we tried to swim with a heavy winter coat on that was waterladen.
Some dogs just have bodies that are more conducive to swimming, as well. My little Peke mix had a very strong chest and front end; but her hindquarters were short legged and not very strong at all, and most of her weight was in the frinthalf of her body. When she was in the water, she could not push with the hind legs enough to keep them floating or pushing her, and she would end up being upright in the water, and just her face sticking out. She was still trying to swim; so it was not that she did not have the instinct to swim, she just did not have the physical ability to do it.
