Most every historical document I've read has indicated rivers and water sources in towns/cities prior to the 20th century(and in plenty of places through it) would mostly be filled with waste and sewage.

So provided it wasn't a dry moat like most it probably also wasn't water you would want to bath in.

hi! you might be interested in a few of the other conversations about moats. Nothing about swimming, but may provide a broader picture of various moat designs and the potential suitability of water for swimming

• How did people fill moats around castles with water?

• Were moats a common structural feature in Medieval castles and if so, how did attacking armies overcome them?

• Would medieval/ancient moats (the ditches with water that surround castles) ever have wild animals such as alligators/crocodiles in them?