My home town would have an annual hot weather drowning in the lakes, and back then it had a lido which was always rammed in hot weather in addition to the town's main swimming pool. This is not a new issue.
However currently access to swimming, especially casual, fun swimming is dire. If there is still a pool, "general swimming" time slots are typically a couple of 1hr slots per day in a timetable rammed with lane swimming, lessons and other specialist sessions and activities. Groups heading off to cool down near open water probably wouldn't divert to the leisure centre instead, but generally poor provision means poor skills and experience, and less likely to maintain swimming ability which is more likely to be a problem when in open water.
Younger teens as a cohort lost out on swimming access around the junior school years with Covid restrictions. My y10 15yo was fortunate to have just completed his school swimming in Feb 2020 which in his case was a useful compliment to private lessons and really gear-changed his ability. The now y9s of his school had a month before lockdown, and another month in the autumn between tiers and lockdowns. My y8 13yo's year lost half of their swimming provision to fill the gap of the year above and there was a noticable gap in entry level and outcome level between the two classes two years apart. This is a school where most have lessons and our slot was between two schools in deprived areas where the ratios of novice, improver and competent groups was wildly different, and in those schools the majority couldn't functionally swim by any meaningful description (e.g. doggy paddle 10m).
At the same time private lessons at best stopped and started with multiple changes in lockdowns/ tiers. Social distancing meant many pools just didn't run access to family swimming for well over a year. At my DC's pool you could private hire half a pool but the cost was prohibative. While my DCs did have interrupted access to lessons, they had no practical access to fun swimming for over 15 months.
Since 2022 increased energy and transport costs have prevented many schools from accessing swimming provision. Some now do a week or two with "pop-up pools" which will be of limited use. Many have just dropped it entirely. Costs to families have soared and what was once a subsidised cheap afternoon out is now prohibitively expensive. This does not bode well for the next cohort of teenagers in the next 5+ years.
Many children just fizzled out of lessons if they had themfrom 2020 and they are now teenagers with poor water skills and knowledge.
Meanwhile open water swimming is booming on social media. Teenagers won't be watching instructive videos from middle-aged women with tow floats and bright bobble hats, they'll be watching shorts/ tik toks of people ploughing in and having fun, much like the sunset over landmark hills trend that does nothing to promote the countryside code and good hiking skills.
Simply saying "no swimming" does not and has never worked. Sites that are relatively safe with good practice are lumped in with toxic quarries and weirs that have specific, inherent dangers. I can think of a few spots that would be better being renamed "bleach lake" and "sewerage falls" to actually put people off and accuately represent the hazard.
There does need to be awareness of how to be safer in water. Gentle entry to acclimatise is a big one. Check entry and exit points. Get your gasp in while your head is above water, not 6ft down from a jump. How to float. What drowning actually looks like. How to safely assist someone in danger. How to read water (particularly moving water near waterfalls and rivers; foamy water with air bubbles is less boyant aside from currents), spotting features like riptides.
I think this is beyond parents as too many parents don't have the knowledge and skills to pass it on, and it's a group-think issue that needs a proper campaign, resources into schools and use social media to access older teens after years of getting good awareness into their heads.