Não tenho 100% de certeza, mas encontrei este site que explica o que fleur de sel é. Diz que
Like other sea salts, fleur de sel is harvested by evaporating sea water. However, to harvest fleur de sel, workers gently skim the top layer of the sea salt from partially evaporated pools, before it sinks to the bottom again. These salt crystals are very fine, light, and delicate, and must be handled with care and exposed to minimal moisture, or they will clump again.
Parece que você está reproduzindo como é feita a flor de sal, fervendo a água, o que equivale a "evaporar a água do mar".