My brother drowned. He was drunk in a canoe with no life jacket and wearing a leather jacket and heavy boots. Knowing how to swim wouldn't have saved him; he cried through his swimming lessons right alongside me.
He was with two friends who just barely survived. One boy had a life jacket, but the girl didn't have one, and she was pulling him under as he was trying to help her. If a fisherman hadn't seen them at literally the last minute, they both would have drowned too.
So it's not just knowing how to swim and having a life jacket; it's knowing how to behave around water and knowing when to stay out of it.