The fact that they were screwed to the floor sounds makes it sound egress would be nearly impossible in an emergency. The way those harnesses are designed there is almost no way to free yourself if they are under tension. You effectively need to call someone else to help you get out.
On top of all that, the mammalian diving reflex means that unless you are experienced and well-prepared you are probably going to gasp as you go under water. Even a strong swimmer can drown due to the effects of the diving response, most commonly in a chaotic situation where they don't have the time or space they normally would to recover.
Of course given the chaos I don't think you could say everyone would have survived with different belts on, but I think it would have been much more likely.
Well, the pilot -- wearing a normal belt -- was the sole survivor; and claims that he tried to disengage the belt of the passenger next to him unsuccessfully before escaping.
from the NTSB preliminary report:
"After impacting the water, the chin bubble on the pilot's side began to fill with water, which quickly covered the floor. He kept his restraint on and reached down for the front seat passenger's carabiner attachment to the helicopter. He turned the knurled screw "two or three rotations"; by that time, the helicopter was "listing past a 45° roll." He then decided to egress the helicopter, and by the time he unbuckled his restraint, he was fully under water."