The most critical thing is that his vision starts to blur, and his head grows increasingly heavy.
The athletes around him also experience similar issues. Someone suddenly slows down, curling up in agony in the water; someone begins to swim erratically, with a strange posture; and someone... simply stops, their body slowly sinking.
Initially, the spectators and media on the shore did not notice anything amiss.
In the triathlon swimming segment, the athletes were focused on swimming hard, with water splashing all around, making it difficult to see clearly.
Moreover, the water quality of the Yamuna River is murky, with extremely low visibility; if someone sinks, it's almost impossible to see from afar.
