47 CFR §80.1121

1. (a)Ship or ship earth stations that receive a distress alert must, as soon as possible, inform the master or person responsible for the ship of the contents of the distress alert.
2. (b)For VHF and MF, ships in receipt of a distress alert shall not transmit a distress alert relay, but should listen on the distress traffic channel for 5 minutes and, if appropriate, acknowledge the alert by radiotelephony to the ship in distress and inform the coast station and/or Rescue Coordination Center. Distress alert relays to “all ships” on these bands may only be sent by a ship who has knowledge that another ship in distress is not itself able to transmit the distress alert, and the Master of the ship considers that further help is necessary.
3. (c)For HF, ships in receipt of a distress alert shall listen on the distress traffic channel for 5 minutes. If no distress communications are heard and if the call is not acknowledged by a coast station, the ship shall transmit a distress relay on HF to the coast radio station and inform the Rescue Coordination Center. Distress alert relays to “all Ships” on HF may only be sent by a ship who has knowledge that another ship in distress is not itself able to transmit the distress alert, and the Master of the ship considers that further help is necessary.
4. (d)In cases where distress alert continues to be received from the same source, the ship may, after consultation with the Rescue Coordination Center, transmit a DSC acknowledgment to terminate the call.
5. (e)A ship station in receipt of a shore-to-ship distress alert relay (see § 80.1113(e)) should establish communication as directed and render such assistance as required and appropriate.