Divining girls need to learn about the spirits near cool springs in the forest, so no, this isn't about Nina-Sensei needing a break on a hot day. Definitely not!
Leave it to Koume to come prepared with bathing suits to solve the problem of seeing through flimsy shrine robes. Kon is the designated cosplay artist, by Koume. Those "harmless" crabs cause a lot of wardrobe malfunctions, even for Saku's deputies.
It's episode 8, so the swimsuit and pool episode was taken out in one go. Nina-Sensei enjoyed her lounging.
Kon worried she may have lost her divining powers because of a taboo ritual. Chiya comforted her the only way she knew how, licking her tears away.