Millwall, Albert & Victoria Docks are all super clean these days, there's a few major events in London that use these for swimming, London Triathlon, Great London Swim and a few others, plus the Thames is swum in by major events pretty much every weekend from April thru September, this weekend Challenge Henley triathlon uses it, I swam in it twice this year at Windsor & Marlow, I have to say as soon as I get out I drink a can of Coke to kill any nasties.