The meaning of Tread-water

phrase

Maintain an upright position in deep water by moving the feet with a walking movement and the hands with a downward circular motion.

Usage examples:

They were at the deep end of the pool and trod water to keep afloat

idiom

To float with your head above the water’s surface and your feet below you by moving your legs and arms up and down

Usage examples:

Stock prices continued to tread water this week.