When we talk about something that belongs to or exists under water, the phrase that comes to mind is "sub aquatic." However, there are a plethora of synonyms that can be used in place of this word. Some of them include "submarine," "underwater," "aquatic," "submersed," "submerged," "below the surface," and "subaqueous." These words can be used interchangeably depending on the context and the tone of the sentence. So, the next time you are writing an article about marine life or water sports, try using these synonyms for a more diverse vocabulary!