BEWARE! All MMSI numbers are not created equal. Those issued by, e.g., Boat U.S. are NOT the same as those issued when you apply for an FCC station license
for your boat. Also, they are not registered in the international list of MMSI numbers.
I had to change mine, having first programmed into my VHF
an MMSI number issued by Boat U.S. My VHF (Standard Horizon) only allows you two chances, so when I programmed in the new FCC-issued MMSI number I did it VERY CAREFULLY. A mistake would have meant I've have had to send the radio
back to a dealer or the factory service
center for programming.
My advice is if you contemplate applying for a station license
(e.g., if you're going abroad or are going to fit an SSB
radio aboard), get the MMSI number from the FCC, not from Boat U.S.