Some countries and states with pool culture legally require a fence around all pools. As far as I can tell, New Zealand has a legal requirement for a fence for over 30 years. There is a little more info on other countries here: https://en.wikipedia.org/wiki/Pool_fence
> I don't mind leaving my child unattended for a short period in a safe area.
...and safety is relative to this child's capability. My parents put me through extensive swimming lessons from a young age precisely so they could let me play unsupervised in the ocean. I'd been a half mile out to sea alone by the time I was 10. Turns out I wasn't as unsupervised as I thought; my mum was freaking out but, unable to swim, couldn't do anything about it!
But if the child can't swim, no alone pool time for them.
Exactly. My oldest son, now 10, has been perfectly able to swim on his own. We live in a former port area with lots of great swimming spots that he visits with his friends. But he's got his swimming diplomas (two of them, which I consider the minimum for this situation).
Sea, though, can be tricky. Half a mile out to sea, currents can be very different. I know that I as a kid once floated on a tiny inflatable boat quite a bit out to sea, and my dad swam after me to drag me back. I thought I could get back on my own, but my dad clearly wasn't entirely convinced.
A true story about an autistic boy and his father who spent a whole night drifting after a riptide pulled them out to sea: https://www.mensjournal.com/features/lost-in-the-waves-19691...
Of course luxury homes with their own pool in the backyard put rather a big strain on safety around your own home. Put a good fence around it, I guess.