The Thames in 1901 served as sewer and trash depository to the city of London, including all the refuse from the many thousands of Industrial Revolution factories. The water the kidnapped girl is set to drown in, and into which Harry dives, would not be clear - it would be filthy dirty, muddy looking, and have lots of trash floating about.
Despite being set in turn of the 20th century London, when the young girl is not breathing after being drowned, Doyle revives her using CPR, which wasn't developed until 1967.