How clean were ancient roman swimming pools?

That depends on how often the bath water were renewed by the slaves in charge of cleaning the pools : if you were rich enough you might have had a somewhat clean private bath (there were about 900 of those at the peak of the Roman Empire) but as Zeenah Iskander explained public baths were dirty and prolific in disease carrying bacteria due to the warm stagnant water. read more

Most Roman baths were also equipped with plunge pools, in which swimming was enjoyed. There are some accounts of women who knew how to swim in ancient times." (this website) Julius Caesar was famous for his swimming ability. Swimming was part of boys' education, and the Romans built the first swimming pools separate from bathing pools. read more