It's a really bad idea to try open water swimming for the first time now. The emergency services don't need the extra work.
There are some organised venues where triathletes and such do their training. These are starting to open up again, on an appointment basis, for serious head down front crawl swim training.
At the coast, there are no lifeguards at the moment.
Normal practice in my area, where we have lots of beautiful rivers, is:
Join a local Facebook swimming page. Search your local river + swimming, see what comes up. Ask on there for advice on locations, local hazards.
Never go alone, always go with someone more experienced if you are inexperienced. This is more difficult now with social distancing.
Learn about cold water safety.
Learn about algae, Weils disease, underwater hazards, safe entry and exit points, warming up afterwards.
Learn how to do your own risk assessment. It's your own judgement whether something is safe or not in the end.
But mainly, don't try your first wild swim until the current crisis is over.