I was discussing anime with some friends and my friend who studied sports medicine/biology explained the significance of each of the Free! characters’ physical builds.
Rei has trouble swimming because as a track runner/pole vaulter, he has really high muscle density and that’s what causes him to sink even though his form is perfect, and to fix this he needs to gain more fat. The butterfly stroke relies heavily on upper body strength though, and this is why it’s the only one he can do.
Nagisa gains an advantage from his height because his short limbs allow him to rotate his arms faster for a faster breast stroke.
Conversely, Makoto’s tallness helps him because the backstroke is a long-axis stroke, so longer arms make him go farther and faster with less exertion.
Freestyle, or the front crawl, is also a long-axis stroke, so again being tall helps. Rin is more powerfully built than Haru and his flip turns boost him a lot farther, but being less bulky makes it easier for Haru to catch up, so they’re pretty evenly matched.
KyoAni really did their homework.