I can't believe this thread. I have been on threads which turn into rucks in my time, but honestly - people snorting with derision because some posters have recommended supervising children while swimming?!
It is common sense to not swim alone. However good a swimmer you are. Cramp can, and does, strike at any time - I can only assume that those of you who are hooting with laughter at the suggestion that people tell their teens not to swim alone have never actually experienced being out of depth in water, and having a serious, debilitating cramp. It is terrifying.
I am not the best of swimmers, but can hold my own - slow, stolid type - not pretty, but good enough. I would not swim alone now, by choice.
My first thoguhts on reading the OP were: hmm, well I did that at that age (my best friend had a pool in the garden - I was over there often, and we were very, very good friends - ie more like sisters. we did not always do exactly the same thing at the same time, and both had free and open access to each other's homes. At the age of 10 (so a little older than the OP's ds, but not much) I would walk to her house (a 10 min walk) or she to mine. At hers, it happened several times that I would go for a swim by myself while she was watching tv. It was all ok. But, having had a few times in my teens where I had severe cramp in a swiming pool - surrounded by people who did not notice, and unable to easily raise attention, I shudder to think what might have happened when I was younger.
Woudl I be comfortable with my dds popping in and out of the pool like I did at that age, with no guaranteed supervision? No way.
I grew up on the beach, virtually. We (my brothers and I, plus children form another family) roamed off on our own form when I was 5/6 or so - eldest child would have been 12, I was the youngest). the only thing we were not allowed to do (we were all sensible children, so there was no chance the others would eg take me out swimming too far in the sea) was swim alone. there had to be a minimum of 3 of us at any one time.
I got complacent when I was older, and at my friend's house. It worked for me, luckily - and now I know better.
I would not let my dds do the same, because of my experiences. Thankfully, they have never been in difficulty in water - they have no idea how easy it is to get into difficulty, nor how terrifying, nor how hard it is to make others aware of your difficulties. I would be happy for this to remain the status quo, and I very much hope it does.