Triathlete vs Swimmer

On what evidence?  

I'd say look at the body fat on most Olympic swimmers and you'll see clear evidence that people with very low body fat can indeed swim quite well!  

Swimming well is a matter of form.  I've seen plenty of people who *shouldn't* swim fast but do because they swam competitively as a kid and learned good form.  Conversely, I've seen plenty of people who you'd think could rock the swim but don't because they never learned good technique.  

I've done a number of triathlons as well as a few open water swim events.  My 50th percentile triathlon pace is about 75th percentile in the open water swims.