Another thing I carry is a flare gun. Orion's 12-gauge basic Alerter kit is absolutely ideal. This all fits in a small duffle bag and gets securely fastened via a quick release - a stainless-steel carabiner - some place on deck where it will be the last thing I grab as I head overboard.
Speaking of man overboard, for starters, if you're solo-boating, it behooves you to have some means of killing the motor should you accidentally fall overboard. While this can be as simple as the lanyard-to-belt kill switch, more sophisticated electronics can kill the engine as well as set off an alarm. The newest - made by an outfit called AutoTether - clips into the ignition switch; all you wear is the sensor. If you go into the drink, AutoTether stops the engine and gives you a fighting chance to get back to your boat (the sensor has a button that can manually shut off the engine as well). As of this writing, the AutoTether is compatible with most major outboard brands. Expect to pay around $300 for an AutoTether with two sensors; extra sensors cost $69.
Another company, MariTech Industries, offers a similar product, the Virtual Lifeline Guardian II, that (unlike battery-operated AutoTether) hard-wires into your boat's ignition system and comes with four sensors. It retails for around $900 for the entire system and thus kills your engine(s).
Raymarine has its own MOB sensors, as does ACR Electronics (ARX 50 is the latest). Mobi-Alert makes a model similar to the MariTech for about the same price, and Sea Marshall also makes a MOB unit.
When All Is Said and Done
If your boat goes down, or is in danger of doing so, and you've done everything you can, the only thing left to save your life is an EPIRB (or PLB, a Personal Locator Beacon). As of Feb. 1, 2009, an EPIRB must broadcast on 406 MHz if you want it to be heard by the worldwide COSPAS (a Russian acronym translating to: Space System for the Search of Vessels in Distress)-SARSAT (Search and Rescue Satellite-Aided Tracking) satellite monitoring system. Units of 121.5 (and 243) MHz will no longer be processed by satellites after February 2009 and are relegated to serve as a homing frequency once rescue craft - alerted by an EPIRB - respond.