The water in Broken River rushed rapidly. If a character stopped moving in the middle of the water, they would certainly be swept away. Zhou Guangyi's Cold Seasons was forced into the water by Stolord's slash. His first priority at this moment was figuring out how to escape Ye Xiu and Fang Rui. Fighting against the water's flow could come later.
Once they entered the water, Stolord and Boundless Sea hadn't stopped attacking, and the two characters had taken control of all the advantageous positions.
What were the advantageous positions? In the midst of the rushing water, upstream positions were advantageous. Attacks that followed the flow wouldn't be as impeded by the water, and these attacks would in fact travel faster than normal. With how swiftly the Broken River flowed, the effect of the additional speed was quite significant.
