Empty water cups can be checked. An empty cup is just a container, whether it is a thermos cup or a plastic cup, it can be checked.

If the water cup is filled with liquid, and the liquid capacity is less than or equal to 100ML, you can directly take the liquid water cup on the plane. However, if the liquid capacity is greater than 100ML, it must be checked and boarded.

Please note that these regulations may vary by region and airline. It is recommended that you consult the relevant regulations of airlines or airports to obtain the most accurate information.