If you are swimming or vigorously treading water, two things will happen that make you colder:
- significantly increased rate of blood flow out to the moving muscles in the arms and legs, which are long and thin and have a lot of surface area to dump heat
- increased heat transfer from your skin to the water, because a constant flow of new cold water will replace the still water immediately next to the skin which has already been warmed by your body. This will happen anyway but happens faster if you are moving.
My understanding is that whether this is a good or bad thing depends on how cold the water is and how much heat you’re generating. An Olympic swimmer training in a pool might prefer something on the lower end of normal pool range (i.e. still not very cold).
I’m honestly not sure in an open water race whether it’s a performance issue, a comfort issue, a safety issue or just something athletes say to psych themselves to go hard. It could be that when you’re swimming hard, the endorphins numb the perception of cold. There’s a chance these effects are non-linear, and the difference in heat loss between floating/swimming is large, but moderate/fast is smaller, and if the difference in heat generated rose with speed in a non-linear way there could be some speed at which the lines of those graphs cross and speed is favorable. If I find a link I’ll post it. But there may not be a rule of thumb; it may depend on exact water temp, race length, and physiology of the individual.
But an ordinary person in a dangerous survival situation would not want this enhanced cooling.