Seems that the requirement being due to the Widener family donation is apocryphal[0], but still an interesting idea. Also, that source suggests that there was a swimming requirement for all students only from fall 1970 to spring 1975. It wouldn't surprise me to hear that it wasn't officially required in 1976 but was effectively compulsory for anyone without a "legitimate" dispension.

[0] http://asklib.hcl.harvard.edu/faq/81791