What is a prefix for underwater?

There isn't one specific prefix for "underwater" in English. However, depending on the context, you could use various prefixes to create words related to being underwater:

* Sub-: This is the most common prefix for underwater, like "submarine" or "submerged."

* Aqua-: This prefix specifically relates to water, like "aquatic" or "aquamarine."

* Hydro-: This prefix also relates to water, often emphasizing its properties, like "hydrostatic" or "hydrophobic."

* Hypo-: This prefix means "below" or "under," and could be used for underwater concepts like "hypobaric" (referring to pressure below the surface).

The best prefix to use will depend on the specific meaning you want to convey.