When I lived in the southwest I did allot of off reading in remote parts of the desert. I used a sattelite transponder from SPOT, it had 2 modes. The first was I need help, the second was the OH SHIT mode for life's in danger. It also comes with 100,000$ of rescue insurance.
Only once was it used, one of our group was seriouly bitten by a rattle snake. We were about 100 miles from the nearest paved road. Within 45 minutes a helicopter showed up and saved his life and didn't cost a dime. 45 minutes = dead in a drowning situation, but worth looking into, my experince is 6yrs old. I'm sure things have changed since then.