In addition to the word AQUA itself, there are at least nine English words that begin with these four letters:

Aqualung, Aquamarine, Aquanauts, Aquaplane, Aquaria, Aquarian, Aquarium, Aquarius, Aquatic

Most of them refer to things that relate to water or its color. The word "aquatic" means that something involves water; for example, an aquatic bird or insect frequently swims in lakes, rivers, or oceans.

Sometimes the word "marine" can be used interchangeably with "aquatic", as in "marine wildlife". Also, "aquifer" sounds like it might be one of the words that begin with AQUA, but it does not.