A couple of years ago, when I was sick with a fever, I remember dreaming that I was swimming freestyle down a long hall in the air. When I wasn't focused on keeping my weight forward, my legs would start to drag on the floor. When I shifted my weight forward, my whole body would rise so that I was swimming close to the ceiling. Does that meet your standards for imaginary swimming?
As good as that dream was, my swim today was even better! Towards the end of the swim, I felt like I had my timing down so that my body axis wasn't moving at all as I rotated. As a result, I needed much less energy in my kick to keep the rotation going, and the whole thing felt wonderful. I wish every day in the pool could feel that good!