Why do shampoos and shower gels list 'aqua' as an ingredient rather than 'water'?

They list it as "aqua" because they are abiding by the INCI (International Nomenclature of Cosmetic Ingredients) naming convention which is a system of names for waxes, oils, pigments, chemicals, and other ingredients of soaps, cosmetics, etc., based on scientific names and other Latin and English words. This way everyone knows exactly what's what when they read the label or look up the ingredients.

https://wayback.archive-it.org/7993/20180908090741/https://www.fda.gov/Cosmetics/Labeling/IngredientNames/ucm2005218.htm

NOTE: The INCI allows for either of the following: Aqua (Water) OR Water (Aqua)