Here's a question to ponder: you end up in the water off your boat floating in your PFD but you're alone or your spouse or crew is unable to get the boat back to pick you up. What one item of safety gear would you want to have with you to increase your chances for rescue?
I found myself thinking about this recently after reading quite a number of reports about sailors in exactly that situation as the result of a wide variety of circumstances. You'd be surprised how often it happens. And when it happens, one of three things will occur: you might be lucky enough to be seen and rescued by another boater, you might die, or you might have had the foresight to have on your person the means to notify rescuers. So what do you want to have with you?
A strobe, flare, or signal mirror works only if another boat comes near enough. A cell phone will likely get wet or lose its signal. A submersible PLB or handheld VHF radio will actively allow you to call for help, however - but in different ways. A handheld VHF with built-in GPS and DSC would be great because pressing one button will send a mayday call with your exact location to other boats and the Coast Guard. But what if you are out of radio range? A PLB will work anywhere, but it notifies rescuers by satellite, which could take a little while - while boats may be passing by less than a mile away. So which of these would you want?
Or both, if you can afford it and don't mind clipping stuff to your belt. Myself, I just bought the latest DSC GPS submersible VHF (photo here) and a small PLB - now it remains to be seen how firm is my resolve to have both clipped securely to my body at all times when sailing. In any case, it's worth thinking about if you like your life!