TOKYO — They are among the Games’ earliest risers and some of its hardiest competitors, waking well before dawn for a race start at 6:30 a.m. that requires diving into a hot, polluted bay that one competitor likened to a “warm puddle.”

For nearly two hours, they knife a ragged line through the murky water and occasionally get hit by fish, until the end, when they thrash furiously to a finish that belies the languid pace of the 10-kilometer swim and often with just seconds separating gold and silver.