It is commonly thought that fat people stay warmer in water. If you asked someone "why", you'd probably here a joke about blubber. It's not that simple, however.
In fact, fat does not keep you warmer underwater. It is actually muscle tissue which generates heat that keeps you warmer. Fat can't generate heat. But again, it's not that simple, so low body fat does not mean staying warmer.
Total body mass (muscle and fat) is more important than just proportion of body fat (although a little bit of fat tissue actually does provide a small amount of insulation). It's hard to say anything definitive about the subject, but those with proportionately more lean muscle mass will tend to stay warmer. However, this can vary with other factors such as what temperatures you are used to as well as your metabolism.
You'll notice that larger people do tend to stay warmer with less exposure protection. Given the above information, I believe that this is because larger people not only have more fat, but generally have more muscle underneath the fat. As I said above, it is the total body mass that is more important than the actual body fat percentage.
Don't take this as an excuse to eat more cheesecake, though. :)