Which of the following can NOT be done while bathers are using the pool?

Using mechanical pool cleaning equipment while bathers are in the pool is not advisable due to safety and operational concerns. These machines can create safety hazards such as entrapment or injury due to moving parts. Additionally, the equipment often requires significant space and may obstruct safe access to the pool area for bathers.

In contrast, cleaning the pool deck can usually be done without interfering with bathers, as long as it does not create slip hazards. Checking water chemistry can also be performed safely while bathers are present since it typically involves taking water samples and does not impede their ability to enjoy the pool. Providing swimming lessons is considered safe as well, as it is a structured activity designed to assist and educate swimmers in the water.