And with this, volume 15 is done. Yes, the entire volume was on this entire arc, and it's the first time we see a clearly non-female akayashi actually taking the combat role (since Shishimarou seems to be... gender neutral.)
@fk1995 france001 is correct. Why would he need to give swimming lessons to a water ayakashi?
Edit: He did give "lessons" to Kawahime at one point, but they weren't about swimming. They were about approaching guys, because she's got the "vitality draining succubus" archetype but isn't comfortable with flirting.
@fk1995 You probably miss-remembered because one of the modern tactics that Kappa taught Kawahime was fetishwear- specifically the "school swimsuit," because both Kappa and Kawahime are pervs. They also brought her to the beach later.