April is Pirate-Themed Supervillain Month
The criminal known only as the Sea-Fox made his first and only appearance in 1960's Batman #132. Besides the point ears and the fox-head emblem on his chest, the Sea-Fox was a pretty no nonsense crook. He realized that Gotham City was built on an island that was positively crisscrossed with underground rivers and tributaries. A couple diving lessons for his crew, and suddenly they could pull off crimes no one else could even imagine with ease. Of course Batman and Robin eventually stumbled onto them, but even with the Bat-Submarine, the Sea-Fox and his gang initially eluded capture on their underwater motorized sleds. Mind you that's a trick that only really works once, and after breaking one of the Sea-Fox's henchmen out of police custody so that they could follow him back to the hideout, Batman and Robin captured the Sea-Fox and sent him off to prison, where he was never seen or heard from again.
Let's start with the bad. The Sea-Fox has terrible fashion sense. It's not entirely his fault, he's mostly a product of his time. But a purple costume with a fox head on it is pretty bad. Mind you, all it really needs is to ditch the emblem and darken the rest of the suit and you've got something to work with. His only other major flaw is that he doesn't really have one. He's pretty bland. It's hinted at that he's got a bit of an ego, but considering the fact it's mostly luck on the part of Batman and Robin that got him caught, it doesn't really seem unjustified. So maybe just play that up a little bit.
In much the same way I like the Rocketeer Racketeers for seeing a criminal opening in the skies and seizing it, the Sea-Fox deserves the same kind of respect. Batman's a guy swinging from rooftop to rooftop, so if you're going to commit crimes in his city, why not commit them by swimming through underground passageways. On top of that he's got some cool gadgets and an abandoned island fortress. That's the makings of a pretty good reoccurring villain. All he really needs to do is contact the Great White Shark to see if the General has any naval henchmen trained. Then just wait for his ego to be his undoing.