What sound signal indicates that a sailing vessel is underwater in the fog?

A sailing vessel that is partially underwater under conditions of restricted visibility such as fog has a choice of using any audible distress signal, or sounding one prolonged blast and two short blasts at intervals of no more than two minutes. This indicates a vessel in distress in fog.

Among the audible distress signals permissible are gun shots fired at the rate of one shot per minute, a continuously sounding foghorn, a radio "Mayday" signal, and radio-telephone or radio-telegraph alarms. In the case of a completely underwater sailing vessel from which the crew has escaped safely, unmistakable light and sound signals may be used to attract the attention of other vessels to indicate that there is danger ahead and a vessel has sunk. The sounds used must not be used for any other kind of signal and must not be able to be confused with other signals.