I don't think there is a perfect solution for the specific problem of shooting macro with a big camera rig and towing a flag. Clipping the flag is unsafe, holding it is going to mess with your shot, and letting it go while you concentrate on the shot is probably not going to end well.
I think the answer is to realize the limitations if you can't find someone else to handle the flag and settle for wide angle for that dive or even... leave the camera behind?
No I didn't say that, must have been that guy over there.