Well, since this is a chemistry question I'm trying to find the right angle to go about, but let me tell you what I think in a simple version of anatomy, then give a half answer in chemistry.
Ok, thinking about it, when people are drowning in water, they tend to be panic which in turn raises the heart rate of the person dramatically, meaning blood is pumping in every which way direction and such. Thus, someone in cold water, their body temperature would drop, as well as their whole body would slow down, since now here is where I apply chemistry, typically about 95% of the time, when temperature is dropped, the reaction slows down and the atoms move slower, thus same idea that your body is a bunch of atoms slowing down, thus you're using less energy and when you get so cold your body stiffens causing your heart rate to decrease.
As, if you were in warmer water, you would waste all the energy in your body trying to survive since you will not get stiff and slow down, you will keep trying to move and the persons heart would go crazy which to have effective blood to the brain the body you need oxygen, the faster your heart pumps, typically it needs more oxygen, and if you're drowning the less oxygen you're getting causing a huge problem.
Thus, colder water you're slowing down, needing less oxygen than someone who is moving around a ton and needing a lot more oxygen, which sounds like a pretty good reasoning why people in colder water are found more often alive than people in warmer waters.
I hope this helps you out a bit, I couldn't think of another chemical reason for this occurance, other than the normal body function requires the oxygen and colder people tend to move less and thus needing less oxygen.
Think of it as the colder water person as someone walking, they breath easier, and the warmer water person as a person running quickly, they're in need of a lot more oxygen, just my thoughts, I hope this help :+)