I'm no diver, but I did some poking around.

There is a nice, quite detailed article on the Nautilus Lifeline here: Alert Diver Online.

Originally Posted By: Tyber
Actualy this looks like a great backup plan, but I would still have another backup plan past that like a PLB that can go diving with me just in case this doesn't work


There's a good summary of the popular diving "backup plan" hardware at the Scuba STAR (Safety Tracking and Reporting) website.

On that site, they present the results of a number of their (quite interesting) diver safety surveys, but the most pertinent one to the Nautilus is here: Search this page for the "DEC/JAN '09 SAFETY SURVEY: Surface Signaling Equipment"

95% of the respondents indicated that they have needed urgent/abnormal pickup at least once (which I infer means they needed to use some of the hardware below). { I think this is an "opt-in" survey, so there is probably some selection bias - those who have had problems might be more likely to respond to the survey).

In summary, the signaling devices the responding divers carried most often (from most to least common) were:
Safety Sausage (nearly universal), whistle, flashlight, signal mirror, air horn and strobe.

What they most commonly actually used (from most to least common) were:
Safety Sausage, (whistle or flashlight) (2-way tie), (signal mirror | air horn | strobe) (3-way tie).

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To indulge in my particular focus: it looks like about 1 in 3.5 signal mirror carriers received specialized training in their use, and no-one reported a signal mirror malfunction or failure.

(There's lots more info in the survey results - I encourage those interested to read the full article to get it w/o my filtering.)
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A signal mirror should backup a radio distress signal, like a 406 MHz PLB (ACR PLB) (Ocean Signal PLB)