Most bodies of water like ponds and canals in urban settings - and even in rural settings - will harbour Weil's disease spread through rat's urine as well as lots of other assorted, poo, gunk and slime. So definitely don full spacesuit IMO before leaping in.
I used to swim in the Serpentine Lido, and unless things have changed, the water in the lido was chlorinated.
As has been pointed out there is the risk of Weil's disease, but to me there is just as much a risk of getting something nasty by swimming in the sea in some areas.
Like the old joke says - a chap fell off the end of Blackpool Pier. He couldn't w swim, but went through the motions.