Most forms of SOS signaling require you to be alive to push the button (there are some exceptions, e.g. some marine beacons automatically activate if you fall into water), but I found you another rationalization for a smartwatch :-) I haven't seen an actual app, but it should exist (or be trivially easy to program): monitor your pulse and if it drops to zero, start screaming its head off via email, SMS, FB messages, dial 911, call the Coast Guard, etc. etc.
I don't recall yet finding a phone-watch with a reliable pulse sensor. That may be because it's a feature I wasn't looking for, but it's also possible that such a product doesn't yet exist.
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