The pontoons are already underwater. The are connected to the upper decks via piping and valves to control air and water into them that raises and lowers the rig.
Prior to the fire, the valving was all closed up because the platform was in a stable position. You could have tripled the fire fighting boats and not affected the water into the pontoons.
But once the fire got so hot that piping and valving started melting, that rig was going down with no fire boats.
The only hope was to put out the fire (nearly impossible) and then try to fix the leaks.
In a fire that big it is almost certainly a lost cause. But fighting the fire is the only hope. Without all that water the temperatures would have been hotter, it would have melted sooner and sunk faster.