Changing your tone of voice to quickly and sharply bark at a child to correct misbehavior.

Establish a positive environment where you constantly give directions and positive reinforcement. Make it clear that a "Bark" is not you being mad at the swimmer, but desiring a different outcome.

Sometimes you can achieve the desired result by shaking your head in a "no" motion.

Sometimes you can just, "Stop that"

Or just say their name in a different tone.

OR "Unacceptable." "stop now."

Move on, and continue with positive reinforcement immediately after you achieve a desired result.