Really good battle. I think Arthur would take it, tho.
First, if Arthur is able to take the fight to the water at all, it's over. He has a massive advantage underwater - he would be way faster, more durable, stronger, etc. to a level that I don't think Tony could compete. I think in most cases in which Tony doesn't end the fight early, Arthur would undoubtedly move the fight to the water upon seeing he can't decisively win on land.
That being said, I'm not convinced he can't win on land, either. We have seen absurd strength feats from Arthur in the New-52 (like picking up a cruise ship, smacking Superman across Gotham, etc.) that would make him incredibly effective. Tony does have the speed/flight advantage on land, however I'm not convinced he has the ranged advantage. While he does have a more versatile arsenal of ranged attacks, Arthur could throw his trident at him (which, with his strength, would be a pretty fast-moving projectile), throw cars at him, etc. Even if Arthur couldn't tag him with those attacks, he could at least use them to help move the fight towards water for a decisive win.