There are three reasons which I can see:
- labelling regulation and a common nomenclature of ingredients,
- ubiquity of the term aqua,
- the romantic ring to the word.
I shall expand upon the first as the others are self-explanatory.
Overview of laws and regulations
The United States Food and Drug Administration (FDA) summarises the Fair Packaging and Labeling Act (FPLA).
If a cosmetic product is marketed on a retail basis to consumers—such as in stores, online, or “person to person”—the ingredients must be listed by their common or usual names, generally in descending order of predominance.
For such consumer products, the FPLA actually prohibits the use of the word aqua on its own.
Cosmetic companies sometimes ask FDA about identifying botanicals only by their Latin names ... such as “Aqua” ... instead of “Water”. Under the FPLA, however, ingredients must be listed by their “common or usual names,” and FDA does not accept these alternatives as substitutes.
They do, however, allow simultaneous use which might be something that makes uniform international labelling easier. Specifically, writing
is allowed. Often times people buy their product directly from professionals, and in this case simply aqua might still end up on your bathroom counter.
[FPLA does not apply] to products distributed only
- as free samples,
- for professional use, or
- for institutional use, such as at schools, hospitals, or the workplace.
In the European Union, the INCI is followed but indirectly as far as I can tell. Indeed, regulation no 1223/2009 (available as a documentation here) states
The Commission shall compile and update a glossary of common ingredient names. To this end, the Commission shall take account of internationally recognised nomenclatures including the International Nomenclature of Cosmetic Ingredients (INCI).
Consult your local EU lawyer for further information. :-)