I grew up on the south coast, and there was also a river on the farm, not to mention troughs for cattle and so on. We were taught about water safety from a young age, and that even the strongest swimmer can get into trouble. I don't remember my first swimming lessons, but my mother thought learning to swim was a high priority. I am quite a strong swimmer, and was involved in lifesaving from age 14. I paid my way through uni on the wages from teaching swimming and lifeguarding.
I love swimming outside, it's a million times better than swimming indoors (at least in summer!) I was blessed in growing up in Dorset, where we have lots of beaches which are good for swimming, and I have really been missing being able to go down for a swim after school/work, now I don't live there.
If you've got the option of using beaches with lifeguards, that's best, but it's not an option everywhere. Learn to know what flags mean, but if there aren't any flags, that doesn't mean either that it's safe or unsafe. Know that beaches all have their own characteristics, dependent on tides and geology. Some slope out gently, others can shelve suddenly, and you can go from it being up to your knees in one step to being out of your depth.
Be very wary if you can't see the bottom through the water. You can't estimate the depth, and can't see if there's anything sharp or anything. Rivers can be safe, but it very much depends on the depth and water flow. Conditions can change very quickly. Aim for shallower, slower-flowing, clear waters, than deep, fast-flowing water, particularly if it's fast enough to churn white. If it's coming off mountains or come through caves, it could be very cold. Be very wary if it's a river with waterfalls or lots of rocks.
Lakes can also be okay - avoid them if they've got much algae visible, though. I would avoid disused quarries. They tend to be very deep, which will make the water much colder. Cold water makes you gasp, which increases the risk of you inhaling water. Your extremities will go numb as your blood concentrates on protection your vital organs, and you won't be able to judge what's under your feet or control your limbs so well. Hypothermia can affect your mental state and judgement, and you may not realise you're getting into trouble. (And in British waters, hypothermia is more likely to kill you than drowning.) Sudden immersion into cold water could trigger a heart attack, particularly if you're not fit.
I also have a vague memory from when I was doing lifesaving exams that something like 25% of all drownings in British waters is attributable to alcohol.
I don't want to put anyone off swimming - it's a brilliant sport, and swimming outside is fantastic, and I've done it in loads of places round the world (including some places in ignorance of some of the risks, like crocodiles, as was pointed out to me after I got out of the water at one place in Malawi... Obviously I survived.) There can also be risks with waterborne diseases like Weil's disease (leptopspirosis), not to mention tons of other diseases and parasites if you're in the tropics, but the risks in UK waters are low (not non-existent - there are cases of leptospirosis every year, and you should be aware of the symptoms, even if the risk is low.)
Are others swimming where you're thinking of swimming? Find out local knowledge if you can. The more swimmers there are, the warmer the water's likely to be, and probably it's safer. Some places will be known as safer swimming spots - check out something like www.wildswimming.co.uk/ There are other websites, but this is probably the best for the UK.
(Sorry for wittering on - it's a talent of mine...)
