Ma'tanza did not honestly expect to overtake Tzirret in a simple grapple, so she had the foresight to take in a deep breath before being thrown under the water. She did not try to stand up immediately, as Tzirret expected, but rather she looked around and noticed him diving under along with her. She did not know his exact intentions, but she was able to twist herself around to face him. 

After managing to get her foot on the riverbed, Ma'tanza pushed off towards Tzirret and attempted to get her arms around him once more. She knew she had no chance of overpowering him, but it was possible she might be able to keep him under the water with her. From there, she was betting on the possibility that she might be able to hold her breath longer than he could. If so, he might be forced to give up, which was the best way she saw of actually winning.