I think they can breath water. They did it in both MW (activated via spell) and OB (innate). I'd guess they'd be almost like a lung fish, in that they have lungs for terrestrial environments, and gills or something similar for water.
And they can't be amphibians. To be an amphibian, they need to constantly moist, no scales, and go through a tadpole-like water phase and a terrestrial advlt phase. Argonians do not need to be moist, they have scales, and do not have two life phases like an amphibian.
And they can't be amphibians. To be an amphibian, they need to constantly moist, no scales, and go through a tadpole-like water phase and a terrestrial advlt phase. Argonians do not need to be moist, they have scales, and do not have two life phases like an amphibian.
Just a nitpick, but Argonians do have a tadpole-like stage. They're born as little lizard things, then they lick the Hist sap, and metamorphasize into humanoids.
I don't think Argonians can really be categorized by Earth terms. They're sort of a reptile/amphibian/mammal* hybrid.
*The females have mammary glands, and they appear to be warm-blooded: I've never seen an Argonian lie in the sun for hours to regulate his body temperature. Not to mention their clearly primate-like bone structure.