I think if I was venturing further offshore...or going into remote places unlikely to have a mobile signal I would invest in a GPS personal locator. A lot of the chandlers seems to stock the McMurdo Fast Find 220 Personal Location Beacon with GPS. It is pretty small, waterproof and the battery lasts 5 or 6 years. It does not have a lot of bells and whistles. If you are in serious trouble and need emergency services you just pull the top off which releases the antenna and press a button. It alerts emergency services with the beacon signal lasting 24 hours. Cost is about 175. My mate who has an offshore rib always wears one on his life jacket.