Yes, there was a gun flash.
My notes on the whole scheme:
-About the Mafia and the Businessman: It's strange that they were talking to each other at the docks. Couple that with the fact that businessman is the one with the silk hat and is missing from the shot, and then with the fact that mafia was heckling the others makes them prime suspects. Remember, this diamond is fabled to bring happiness to whoever owns it. But then again, Businessman could also have been pushed off of the boat, so...
-Mechanic: Possibly, but I would guess that she was the one who turned off the lights, so almost sure of her being one of the people in the plan.
-Politician: Wine was also what I was thinking, there was only one gunshot and it's stated that he'd spilled his wine before. There are a few possibilities here: 1. He actually has a condition. 2. Doctor is guilty, drugged him or something. 3. Waitress and Actress are in cahoots, also part of this whole plan. Waitress is the one who poisons the wine and champagne, and since Actress knows this, she orders whiskey on the rocks instead
-Actress: The dropping of her glass brings on a few theories, that maybe she had the diamond in the glass, pretending it was ice (whiskey on the rocks) and dropped it for the waitress to pick up. Or maybe Waitress accidentally dropped the diamond and Actress dropped her glass to disguise the diamond and have reason for the waitress to bend down and pick it up. Mafia might've seen the fumble and laughed at that.
-Police: Probably involved, only one who could have a gun without being suspicious. Keeping an eye on the mafia because she's scared that he knows? Or maybe he's part of the plan too and she thinks that he might betray them all or cheat them with the businessman, who he was conversing with earlier.
-Auctioneer: Eh, not much to say.
-Magician: Forgot to mention, she actually has a great spot to steal the diamond, as she's on the stage with it. Possible the one who stole it, hiding it by sleight of hand. Or maybe, in the possible scenario that they're all in cahoots and Mafia and Businessman cheated them out of it, the one who was supposed to get the diamond. Looks very shocked.
-Journalist: Could've hidden it in her camera, has the bullet at her feet.