Couldn't sink

In Lotus Eaters Bloom recalls a photograph (there are several variants of the same scene, taken in 1900) of a man "floating on his back, reading a book with a parasol open." He is correct that you "Couldn't sink if you tried: so thick with salt," but he cannot quite recall the "law" responsible for this result.

JH 2018

Around the Dead Sea, 1900 photograph held in the G. Eric and Edith Matson Photograph Collection, Library of Congress. Source: Wikimedia Commons.