Before you can use the digital selective calling feature of your VHF radio, you need to use www.boatus.com/MMSI to get a Maritime Mobile Service Identity assigned to your vessel.

Once your MMSI number is assigned, you need to program it into your radio, according to the instructions in your radio manual. You also need to connect your GPS to your radio. Once you've done that, when you press the Emergency Call Button on the radio, the radio will tell the Coast Guard who and where you are, so they can help you.

Without that MMSI number, the Coast Guard has no idea whether this is a ship, a 16-foot runabout, a 50-foot sailing yacht, a canoe, or what. They also do not know where to look for you.