There are two map of UK (and a few Irish) open water swimming locations.
On wildswim.com you can add locations. Dover isn’t on it yet for the UK, how ridiculous is that? And there’s about 4 locations of Ireland. This is just a screenshot. Original is here. If you can be bothered. Though I do like the colour-coded location markers to indicate what type of swim it is.
And there’s also wildswimming.co.uk, claiming to be the larger of the two sites, which in fairness is a better site. Also with a map AND the book, thought he map isn’t any better for us sea-swimmers. It seems to be more about rivers and lakes and pools.
The British (hi honey!) are gone mad on the whole “wild swimming” thing, which would be a nice title if it hadn’t been completely compromised by the BBC having Alice Roberts swim granny stroke in a 14C pool telling everyone how cold was and ITV and Robson Greene doing a not-as-cringeworthy swim in the Thames in Henley and making a big deal of a red pair of budgie smugglers. I dunno what they’d do if they ever had their eyeballs assaulted by one of Ned’s disappearing togs events.