Interesting, I wonder how dramatic the effect is with swimming, there are swim flumes like this https://www.youtube.com/watch?v=feUhyCklHL0. It's usually used to analyze your stroke, but maybe if you put a bar in there to hold on to and test unshaven vs. shaven.
As a former swimmer, I can say that shaving is not only done for drag. Once shaved and in the water, even while casually swimming in warmup, I would feel as if I'm actually floating better. We'd call the "feel for the water" or something similar. It's hard to describe unless you've experienced it, but I suppose it has something to do with a newly exposed layer of skin in contact with the water.
And, of course, there are also the benefits of less drag and psychological placebo effect.
As a former national-level swimmer, I always shaved my legs/arms and tapered my training for a big meet twice a year.
I found that combination to lower my times by about 1 second per 50m, or about 5%. I don't know how much of that is attributable to the shaving, but I will say it helped me "feel" way faster.
