Related Clues:

• 'Sending out an ___' (much-repeated line in a Police hit)
• Emergency call to the Coast Guard
• Desperate plea
• *Rihanna, 2006
• Distress-signal letters
• Help-wanted indication
• 'Mamma Mia!' song
• "Gilligan's Island" signal
• Kin of a 911 call
• 'Help!'... and a hint to 17-, 27-, 42- and 53-Across
• "Mamma Mia" number
• 911 on the ocean
• Urgent transmission
• One might be picked up in a storm
• Sailor's "Help!"