There isn't a specific, universally accepted term for rock chimneys underwater. However, depending on their formation and location, you can describe them using terms like:

* Sea stacks: These are tall, isolated rock formations that rise from the seafloor, often found near coastlines.

* Underwater pinnacles: These are pointed, spire-like rock formations that rise from the seafloor.

* Submerged cliffs: If the rock chimney is part of a larger cliff system that extends below the water's surface.

* Hydrothermal vents: In the case of chimneys that are formed by hydrothermal activity, these are called hydrothermal vents. They are often found in deep ocean environments and are characterized by the release of heated water and dissolved minerals.

Ultimately, the most appropriate term would depend on the specific characteristics of the rock chimney in question.