Black Manta is traditionally an Aquaman villain, although he has faced off against the Green Arrow as well. He wears a suit that allows him to survive underwater, and also gives him enhanced strength. Hes highly intelligent and skilled at hand-to-hand combat. His story doesnt necessarily require Aquaman to work, so he could be introduced on Arrow, and having a threat from the sea would be something very different to what weve seen so far, and could even tie into Olivers time on the island.