The word "underseas" can be a bit limiting when it comes to describing the vast and varied world beneath the ocean's surface. Fortunately, there are a number of synonyms that can help add depth and nuance to our descriptions. For example, the terms "submerged," "subaquatic," and "submarine" can all be useful when referring to things beneath the waves. "Deep-sea," "oceanic," and "pelagic" are other options that can help convey the vast and sometimes unknown nature of the underwater world. Meanwhile, terms like "nautical," "maritime," and "naval" can be useful when talking specifically about activities or vessels related to the sea.