How flexible are centaurs at the waist? Could one lean forward such that the human torso is a nearly straight extension of the horse part? This would provide two benefits: it would lower the center of gravity so more of their body weight would be supported by the water, and it would allow them to stroke with both arms and legs.
I think a centaur can use the flat palms of his hands to hydroplane over the surface, which would give them an edge over unicorns. I guess mermaids might do the same.
I have found artistic evidence!
Unfortunately, swimming was not covered in this interview, but it does contain many other facts which may be of interest to you.
