He could actually swim very well. I figure a heart attack or something along those lines.
I read it in interviews with him and also heard it in an NPR interview.
Swimming was an escape for him. (his own words).
Looks like for whatever reason it was his final escape.
Almost happen to me when I was a kid...got caught in a rip current....luckily I remembered that you can get out of it by swimming side-ways out of it and not struggle trying to swim straight back to shore.