The phrase "deep water" typically describes water that is significantly below the surface, commonly found in oceans, lakes, and rivers. However, there are numerous synonyms that can be used to describe this concept. For example, you could refer to "abyssal depths," "oceanic depths," or "fathomless waters" to convey the sense of the vast and mysterious qualities of deep water. Alternatively, you could use more specific language, such as "submerged depths," "underwater trenches," or "bottomless pits," to describe particular features of this environment. Ultimately, the usage of these synonyms depends on the context and the intended meaning of the phrase.