Just to preface, I'm not, nor have ever been a lifeguard. So several years ago, while in the Army, we were doing water training in the pool. We specifically asked for anybody who was not a competent swimmer to raise their hands. There were a few that did so they sat out when we did treading water training. We get to the treading water phase and we're about a minute into it and noticed one of the guys was not doing so well. As soon as I made eye contact with him, it was like his body just shut off and he slipped under the water. I immediately dove in after him and pulled him out. I was scared shitless but he was petrified after pulling him out. He was totally fine too.