If you hear a distress message from a vessel that goes unanswered, you must answer. Experienced captains write down coordinates if they hear a pan-pan or a mayday, and they are ready to stop what they are doing and come to your assistance if in range.
The radio procedure is always to be scanning Channel 16. If you hear a distress call, press the 16 button, which locks in that channel. Then grab your logbook and pen, ready to record any information relayed from the radio. In addition, you should mentally try to locate the mayday site regarding your own position and prepare your crew and boat to immediately head in that direction if it is nearby.
Even in dire circumstances, bringing a second boat to the scene can make a huge difference. Each year, many on-the-water emergencies are assisted by good Samaritans who arrive on the scene before rescue crews.