"Doctor Doom! We meet at la- never mind! Well, Commissioner, this Doctor Doom character is either an average to below average swimmer or he's dead. Or he could be this shadowy shape right below the water's surface. Call me if you need any other super crimes solved."
While the superheroes and police reminisced about their second long battle against Doctor Doom, he climbed out of the water and hid in the mummy case he stole. And since it was only a priceless relic and the very thing they were working to recover, Commissioner Gordon gives it to Batman to "remind him of his triumph." This was about the craziest thing possible for a cop to do, but to be fair he was probably being sarcastic. Batman's "triumph" was getting a phone call to come to the pier and doing nothing while a man killed himself. That's kind of like helping a brother and sister get their cat out of a tree and then leaving with the top half of the girl.