Sculpture of a mermaid found at a Roman fort near Fors Abbey at Bainbridge in Yorkshire, England.
One of my favourite books about swimming is Haunts of the Black Masseur : the Swimmer as Hero, by Charles Sprawson. I love his chapter on 'Classical Waters' (in Ancient Greece and Rome).
Sprawson tells us that in ancient Rome swimming was associated with sensuality and, eventually, as Emperors built ever more luxurious pools and baths, decadence. The Christian church filled the sea with imaginary monsters.
For Pliny the mermaid was a exciting proof of nature's diversity, her song an irresistible celestial harmony.
For the medieval church her siren song became the lure of fleshly pleasures to be feared and avoided by the godly. Swimming, like sexual pleasure, came to be associated with the devil and was almost suppressed during the domination of Europe by Christianity. Not until the beginning of the 19th century was its popularity revived.