I saw this recently in the Evening Standard – it’s an idea for swimming pools in the Thames in central London.
It looks amazing and it would be a great way to enjoy the river, but without having to fight with the boat traffic.
The concept is that a new sewer system will be employed soon, making the river cleaner and so better to swim in. The pool would be separate to the river in an enclosed tank, but it would fill with water from the Thames at high tide.
A while ago I wrote about an idea to create swimming lanes in London’s canals, it would be amazing if some of these ideas came off and we had much more “wild” swimming happening in London.
UPDATE
There is a Twitter account set up to promote this project – you should follow them here. You can also see much more about the project, with some original designs here.
1000kmstowindermere 
Something similar, pools which are filled by the ocean, is common on the beaches of New South Wales near Sydney (and probably elsewhere). My parents were in sheltered accommodation in Avalon at the end of their lives, here is the one there:
and more generally
http://www.sydney.com/things-to-do/beach-lifestyle/sydneys-outdoor-pools