Where did the factoid that he was a proficient swimmer come from?
Good question.
It's an unusual word to use in this case, as well as "avid" and "enthusiastic."
On the day it happened, I saw a news report that showed a video of him taking swimming lessons and saying "progress..."
It sounds to me like he was "enthusiastic" about learning to swim, he was "avid" in taking his lessons, and he was "proficient" at swimming like JFK Jr. was "proficient" at flying an airplane to Martha's Vineyard.
-PJ