Three fishermen in Palau were 36 hours overdue in their motorized fishing boat (engine trouble?) and two of them decided to swim (8 miles) to land.

Fortunately, a helicopter found the boat and learned of the swimmers from the third fisherman.

A Zodiac boat looking for the swimmers saw their signal mirror and rescued them on Christmas, 12/25/2015.

The link below is the only news story on this I have found.

Palau swimmers flag down Zodiac with Signal Mirror on Christmas
A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)