its not usually recommended due to the low water temperature, but it is an activity that has always taken place, and last week the water was particularly warm, there have now been several organised events on the waveney. it is indeed an activity that every boater should be aware of and watch out for, much as you need to do for canoeists, and paddle boarders, that are out in prolific numbers at the moment, indeed there is no real difference between a swimmer and a paddle boarder that has fallen off their board.
indeed the Broads Authority strongly advise against it but it is allowed.
https://www.broads-authority.gov.uk/boating/navigating-the-broads/outdoor-swimming
but also advise all of the dangers.
if steps are taken to mitigate the risks- eg only swimming when the water is warm enough, avoiding busy areas or times, etc, then swimming may be undertaken with a degree of safety