If Zhang Heng had encountered this huge wave before his trip to the island, he probably wouldn't have had any other choice but to retreat. However, now that he had grasped the basics of controlling the water flow, he could still handle this small situation, although he might not be as powerful as the deepwater overlords like Jemengard.
Zhang Heng used his ability to control water to split the wave into two, brushing past his body and hitting the trees and plants behind him. In the end, the flowers that had been blooming were all destroyed by the oncoming wave. However, Zhang Heng, who was standing in front, was safe and sound.
The black python was obviously surprised as well. Although it didn't seem to understand how the human before it could separate the wave, that didn't stop it from doing what it was about to do next.