If you're looking for a low-maintenance, economical device that will get your coordinates to authorities in a last-resort situation, ACR's ResQLink+ (2) and McMurdo's Fast Find 220 (4) are good options. Both beacons are waterproof, require no service subscription and guarantee a battery shelf-life of six years. The disadvantage is that these are all-or-nothing devices, leaving no room for communicating with search and rescue beyond the initial call for help.
The Fast Find 220 comes with a floating pouch and has the lowest price tag of the four devices tested ($249, mcmurdomarine.com). It's a single-use device--you snap off the red cover to expose the antenna and the "on" button, which will activate your distress signal. Meanwhile, the ResQLink+ floats on its own, and ACR will replace it if used in an emergency situation. But be careful: The button with the power symbol sends the distress button. If you turn this unit on, the helicopter is on the way (acrartex.com, $280).