Which piece of equipment is essential for signaling to other divers while underwater?

The essential piece of equipment for signaling to other divers while underwater is the underwater communication device. This device allows divers to convey messages effectively without relying solely on visual signals or gestures, which may not always be easily interpreted in the underwater environment. Communication devices can include technologies like underwater radios or full-face masks with integrated communication systems.

Other options, although useful for diving, do not serve the specific function of signaling. A snorkel is primarily used for breathing at the surface and does not aid in communication underwater. Fins enhance mobility and speed while swimming but do not facilitate communication. A weight belt is important for adjusting buoyancy, ensuring proper balance in the water, but it does not contribute to signaling. Therefore, the underwater communication device is the only option that directly addresses the need for sending signals to other divers while submerged, making it the correct choice.