As someone that practiced swimming for over 11 years I can vote for doing this.
The water you have in your drinking bottles is generally a bit cooler (and drinkable) than the water you are swimming in (due to swimming in outdoor pools and/or the bottles not being actively being heated up) and after tough swimming session it would be extremely refreshing to splash a bit of clean, cold non-clorine water on your face. You would be surprised how much heat you can build up, even in relatively cool water.
So no, not actually as "redundant" or "defeating the purpose" as non-swimmers might think. We always did this and this helped us cool ourselves down without having to leave the swimming pool.
