More than a dozen scuba divers dot the pool’s turquoise waters, swimming in groups of threes and fours in dark wetsuits and face masks. Excited chatter, splashing, and the occasional squeal of delight pierce the humid, chlorine-scented air, a contrast to the frigid conditions outside on this January morning. To the uninformed observer, the people in the pool at a high school in suburban Chicago are indistinguishable from one another.