Jet A weighs 6.75 lbs/gallon, water weighs 8.34. So yes, once the full fuel bladders are under water they will float.
I think it is logical to assume they could not access the fuel in the bladders. I don't know if they were able to empty them, but I'm assuming they were not. I could be wrong.
I think the plane is floating upside down because the main fuel tanks are empty. As others have pointed out, I was wrong when I assumed there were fuel tanks in the wings. The main fuel tanks are in the belly, under the floor.