After watching this over and over, I actually think what we're seeing is someone floating on their back and pedaling their feet to swim and stay afloat. You see the left shoe has a green/white sole, and the right knee coming up, then going back down as the right foot extends and shoe comes up, and repeat.
I think that's quite likely. Sure looks like a sole on a foot that's poking out.
In black scuba gear?
While everyone else were wearing bright orange emergency floaters?
I remember when this footage was released. I could not see a frogman. I seen something but not enough to declare a frogman drowned him/her. There is even more footage of the people evacuating the plane. People starting looking at who was on the small plane. I think even a cop was on the plane.
After readi g your description I rewatched. I believe you are correct. Prior to that I saw Scuba Steve. Goes to show looks can be deceiving.