"You have to take your clothes off," Dean said. "Don't worry, we won't look."
Sam side-eyed his brother. "There's a locker room, change in there. I found one of these." Sam tossed Cas an ancient wool bathing suit. "Why do you want to swim, anyway? You can poof yourself anywhere."
"It might be...enjoyable." Cas looked doubtfully at the shimmering water of the bunker pool. He went into the locker room, emerging a millisecond later in a baggy 1920s onesie. Ignoring the brothers' derision, he dipped a toe into the water and jumped back. "It's cold."
"It's supposed to be cold, Cas," said Sam. "Go in the shallow end."
"This will not be enjoyable," Cas muttered, going to the ladder at the shallow end. Concentrating, he lowered himself halfway into the water and stopped. "This is definitely not enjoyable."
Dean snickered. Sam jumped into the pool, fully clothed, and walked over to the startled angel. "This is how Dad taught us to swim." He picked up the angel under the arms and threw him into the pool, closer to the deep end. Cas sputtered, but could still stand up.
"Come on, boy, swim, that's an order!" Dean barked.
"You WILL swim!" Sam mimicked Dean.
"And don't poof out of there!"
Sam walked closer, his flannel shirt trailing on the surface of the water like a cape. "Come on, Cas! You can do it."
"You're too tall to understand," Cas snapped.
"Do one of your equations of body weight versus the weight of the water etcetera," Sam said patiently. "Come a little farther out, I'll teach you how to tread water."
Suddenly Cas was no longer in the pool, he was on the surface of the pool, looking down at Sam. "I already know how to tread water."