I know it’s complex, but the
I know it’s complex, but the simplest is just a torpedo…and what fun is that…we need complex to make it intesting, no? 
Or, put one of
Or, put one of these: http://www.findmespot.com/ into a bottle, set it to find its coordinates every few minutes, and post the longitude/latitude to http://www.geocaching.com/ for others to seek out and find and sign the log book / put little doodads into. The only probably is that it would probably be stolen. 
wow, looks like for 199€ +
wow, looks like for 199€ + 99€ annual you can send an unlimited number of email with coordinates via satellite from anywhere in the world… possible ??