For what we can assume is the first time ever, the water is working against Haruka instead of with him.
The boy who knows the water best, who seems to “understand” the water and can move through it so easily is now being pulled back and drowned by it, also like a betrayal.
The fact that something that was once so simple and easy for Haruka is now becoming something he can’t momentarily do is probably, by far, one of the worst things that could happen to him because the water/swimming has up until this moment done nothing but help him and benefit him. The fact that after this moment, he stops racing all together and simply stands there in defeat, it’s easy to assume that this was his breaking point, where he knew that his fears and doubts and thoughts have become too much for him to bear.
This is something Haruka mostly lively never imagined to happen, and the fact that something that was so natural to him is now becoming a challenge would be enough to make anyone stop, even momentarily.