Bathysmal is an interesting and unique word that means "relating to the deep ocean" or "profoundly deep". However, if you're looking for some synonyms to mix things up, there are a few options to consider. "Abyssal" is a close synonym that means "relating to the depths of the sea", and is often used to describe the deepest parts of the ocean. "Deep-sea" is another option that simply means "pertaining to the depths of the ocean". "Subaqueous" and "submarine" both refer to being beneath the surface of the water, but don't specifically connote extreme depth like "bathysmal" does. Finally, "profound" and "deep" are more general synonyms that can also apply to the ocean.