No they don't look like fish, although fins they do have, http://images.uesp.net//2/25/Argonian_official.jpg are fins if I've ever seen one.
Okay, I guess I forgot those, still, I can't see those being used for swimming...
As far as the information, I wonder where the bit about gills comes from? Because I certainly don't see them in any of the games, is it mentioned somewhere?
Now the swiming animation is reflected in Morrowind, though I always thought Bethesda was going for something more along the lines of a crocodile or some lizard capable of swimming than any sort of fish or amphibian. And when it comes to breathing in water, I actually always felt that didn't make much sense precisely because they seem more reptilian than fish-like, granted, they're often described as being equally at home on land and in water, but that could just mean that they can move just as well in water as on land, after all, whales are perfectly at home in water (Though they can't survive on land.) but they still need to surface to breathe. I could see Argonians being able to hold their breath much longer than other races, maybe, but actually being able to breathe under water just always struck me as odd. But hey, that's the ability Bethesda decided to give them, I guess water breathing was just easier to handle using the game mechanics than actually letting their breath meter last longer.
Of course, either way it isn't really relevant to the subject of werewolves, since whether they're closer to reptiles or amphibians, they're still pretty different from wolves, I just don't think that needs to matter much when dealing with beings already removed from real life logic.