We live out in California and the "drowning season" has started. We start hearing about kids who drown in their backyard pool. It's so sad and generally preventable.
I think, ideally, all children would learn how to swim.
Private lessons are usually the most efficient way to get them swimming. My 6 year old was terrified of swimming- we started at 3/4 years old and he would not get in the pool with me or the instructor! They finally gave us our money back for the lessons, saying they "never" had to do that before.
Eventually we got him to try a little bit. Fast forward 2 years- this weekend he won 3 medals and the 6-U boys high point trophy on the swim team! He's so excited. So keep chipping away at swimming.