Once you start swimming outdoors you soon realise it is not straightforward. You find you are coming up against issues such as safety and access.
Link to the Outdoor Swimming Society inland water access guide, and some links on access and legality:
An article on swimming safely in rivers and lakes:
An article on swimming safely in tidal rivers and estuaries:
Swimming in Tidal Rivers and Estuaries
The page, Swimming in the Norfolk and Suffolk Broads allows you to download notes and advice on swimming in tidal rivers, getting started in open water, and places to swim in the Norfolk and Suffolk Broads.