Maybe a place you have to get to from underwater? I don't see Argonians actually living their lives under the sea.
Much more plausible. Therefore, easier to reach for Argonians, and harder for humans and mer, unless they possess a water-breathing potion.
Maybe, could be near a geyser.
Geysers erupt to due extreme pressure produced from extreme heat. Argonians, like every other race, would die.
I hope I interpreted your comment correctly...