Water backing up in your tubs or showers is a common indicator of a main-drain obstruction. This occurs when you try to drain water but there is nowhere for it to go due to a blocked sewage pipe. The water then flows backward in search of the lowest point of entrance. In most homes, this will be the shower, tub or basement floor drain.
The water will not be clear. Because this water is coming from the sewer line, it is generally blackish, smelly and gross-looking. Keep in mind that this water may include raw sewage, so proceed with caution. When cleaning, use appropriate safety equipment and use strong cleaners.