Why do the lifeguards at swimming pools not allow people to rest against the waterline?

The water line is used to divide swimming lanes, not for rookies to drag and rest. Unlike competitions, in public swimming pools, one lane is used by swimmers from both directions. Keep to the right, with the waterline to the swimmer's right. You'll find swimmers very close to the waterline. If you pull the waterline down, the waterline will sink. People swimming breaststroke on both sides of the waterline will step onto the waterline, which can range from pain to contusions and fractures to their toes. Anyone who has been criticized by a lifeguard should reflect on it and see how much of a threat you pose to others.

Do not press the water line. To be precise, it should not be allowed to press the water line to a lower position, such as completely pressing it into the water. The reason is exactly what the top-voted answer said. The sunken waterline can easily injure people swimming near the waterline. I have been hit by the waterline and injured my hands and feet many times. It was really painful. When I stopped, I saw that someone had lowered the waterline! Because swimmers assume that the waterline floats on the water, there are no obstacles below the waterline, and it is impossible to observe the waterline position at all times during swimming. If there is only one person swimming, it doesn't matter how high the waterline is, as no one will be hurt anyway. Of course, this does not mean that the waterline cannot be touched. As long as the waterline is not lowered, it is theoretically allowed to gently hold the waterline, because it will not hurt anyone. However, in this case, the lifeguard is not sure whether you will continue to press hard on the water line, and may still remind you to let go of the water line. Finally, I suggest that the subject swim along the edge of the pool and hold on to the shore at any time. The waterline is really used to separate swimming lanes, but it is not allowed to be used in this way because it is easy to hurt people if pressed. This can be said to be a common rule of swimming pools, don't press the water line. By the way, I am not a lifeguard, but I have been injured by the waterline many times, so I couldn’t help but answer. I remember that when I went swimming in a swimming pool for the first time, I was injured more than once by the lowered water line. Then I saw the sign prohibiting lowering the water line and understood it instantly.