Swimming was a phobia for my older son -- probably because when he had lessons at the "Y", he decided he was a great swimmer and jumped in the deep end without my husband being there, and of course, dropped to the bottom like a lead balloon. Yeah, I'd say that was traumatic. He can swim, but hates to do it.
My younger son swims just fine. His swimming movements are not "free" and natural, but he keeps himself afloat fairly well.