A free-thinking seventeenth-century Frenchman by the name of Monsieur Thévenot published a book, in 1696, entitled L’art de Nager , the English translation, The Art of Swimming, being published in 1699.
The book is, indeed, a treatise on swimming – an illustration and description from which appear below…
To Tread Water
By this way you remain upright [plumb] in the Water without making any motion with the hands, only you move the Water round with your Legs from you, the Soals [sic] of your Feet being perpendicular to the bottom ; you may make uſe of this if you are caſt into the Water bound hand and foot.
When not being harried by unknown assailants and cast into the water, Monsieur Thévenot found the time to invent the bubble level, circa 1660.
 Melchisedech Thévenot, L’art de Nager, Demontré par Figures. Avec des Avis pour se Baigner Utilement, Thomas Moette, Paris, 1696.