Toradora! Volume 4 (Manga) — Yuyuko Takemiya

NAVEL BATTLE!

After endless strife, Ryuuji and Taiga have finally found some common ground and can at least tolerate each other (sort of). Their relations may be on shaky ground, but it's their disdain for popular teen model and recent transfer student Kawashima Ami that unites them.

The battle with Ami has moved to a new front: swim class! A tasteless insult cannot go unpunished. There will be war (in the pool) and the winner will take Ryuuji as her spoils.