Assuming the movie versions? Well, we don't have a huge amount of useful data for Aquaman yet, but. . .
Arthur takes it, after a long, hard-fought fight. What it boils down to is, Aquaman seems to be at least as bricky as Iron Man, maybe moreso, only he doesn't have his powers tied up in a suit of armor that can be attrited over time. Arthur is also a more skilled and agile fighter than Tony.
Ironically, I'd give Steve better odds if you gave them both their weapons. Not only does the shield make it a lot easier for him to play defense, but it opens up the possibility of somehow disarming and getting his hands on the trident, thus having a weapon able to meaningfully hurt Aquaman. As is, he's fighting someone he can't really hurt, and who will eventually hit him.