If she could float, then she wouldn't have been able to dive after him.
So "real reason" is probably more like her body still stronger and more toned for why he could swim, and his body thinner and reacted poorly to the water (cramping up?) surprising her and sinking her pretty quickly.
(Another thing that may or may not be related to the above topic - back when I was a skinny pup I couldn't dive at all, because I would take the deepest breath I could and didn't have the technique to be able to pull myself down against that buoyancy.)