Which signal must all vessels conducting underwater activities use, according to the COLREGS?

According to the International Regulations for Preventing Collisions at Sea (COLREGS), vessels engaged in underwater activities are required to display a rigid replica of the International Code Flag "Alpha." This flag is specifically designated to indicate that a vessel is conducting operations that may limit its ability to maneuver, alerting other mariners to exercise caution in the surrounding waters.

Displaying the Alpha flag is crucial as it serves both as a visual signal to nearby vessels and emphasizes the need for awareness of potential underwater hazards or divers in the area. The requirement for the Alpha flag reinforces safety protocols and helps to prevent accidents by ensuring that other marine traffic is informed of the ongoing underwater activities.

The other signal types mentioned do not fulfill the specific regulatory requirement set forth in the COLREGS for underwater activities. A light signal or sound signal may be applicable in different maritime contexts, such as navigational signaling or warning other vessels, but for the explicit situation of operations under the water where visibility might be reduced or compromised, the rigid replica of the Alpha flag is the designated marker that conveys the necessary information effectively to other vessels.