On youtube i have seen detailers wash their cars under direct sun very easily, the water wasnt drying and they were pretty easily drying their car. Try that over here and you'll have dried up water and water spots by the time you go and get your drying towel.
Under the sun, the body gets really hot especially with the coming summers, so as soon as some water touches it, it will evaporate leaving the minerals behind to etch water marks. Plus i have noticed that when you drying your car after washing under the sun, the rate at which water is evaporating is much more than the rate at which your drying the car, resulting in basically you just rubbing the cloth over the dry body and inflicting swirls. Happened to me, thats why i never wash the car under direct sun. Also that is the reason that most of my washes are at night.
I advise that if you get some partial shade, clean the body under the shade, and the switch the car to cater the uncleaned part. But the best time to clean if you get sunlight in your garage is to clean at night.