We saw er patients occasionally with hyponatremia secondary to water intoxication, stemming from a psychiatric diagnosis of one sort or another. They would present in apparent good health, no muscle cramping, perhaps some anxiety, but with serum sodium levels approaching the 120 meq/l level that may result in seizures. See reference here. . These were cases of chronic excessive ingestion of tapwater, so the picture might be a different in acute hyponatremia due to exercise.