Do I qualify?

Marine vessels fall into two basic categories under GMDSS rules when it comes to the use of Maritime Mobile Service Identities: those vessels which must adhere to international regulations ("compulsory" vessels) and those that don't ("voluntary" or "non-compulsory" vessels). The legal language at the bottom of the page spells this out in detail, but the short story is summarized here.

Vessels used solely in domestic US waters may get a MMSI from this site, such as:

recreational boats
towboats and tugs under 600 horsepower
state and local government vessels.

Under the terms of international treaty, the Federal Communications Commission has jurisdiction over US flagged vessels. The FCC has elected to maintain the issuing authority for MMSIs for all vessels subject to international law including commercial ships, and leisure craft operating in international waters (note that this includes Canada, the Bahamas, and Mexico!).

In summary, if you are traveling internationally, you need to file Form 605 with the FCC to obtain a Ship Station License, which comes with a MMSI. You do not qualify for registration under the program administered within this web site.

How many MMSIs should I get?

One.

If you use any digital transceiving equipment on your boat, a MMSI is a vital part of the data stream.

The following list shows the typical equipment available to the recreational mariner.
If you have any or all of the following devices on the same boat, all units must be programmed with the same MMSI:

VHF-DSC (VHF with Digital Selective Calling)
MF-DSC
High frequency radio with digital capability
AIS (Automatic Identification System)
INMARSAT telephone system.

Also, if you register for and receive a MMSI through this web site, then obtain a Ship Station License later,
yhe MMSI on that license becomes the governing number. You must delete the record in this database
and program your transceivers with the FCC-issued number.
The idea is simple: one boat, one MMSI.

EPIRBs weren't mentioned. Why?

EPIRBs, PLBs (Personal Locator Beacons), and ELTs (Emergency Locating Transmitters) are registered in a separate database maintained by NOAA.
These devices are different because they operate in transmit-only mode.

The NOAA registration service will ask for your MMSI, while the MMSI registration service will ask for your EPIRB code.
To resolve this Catch-22, we recommend that you go to the NOAA web site first and register your EPIRB,
then complete the form on this web site to receive a MMSI.
Ultimately, it is the mariner's responsibility to assure that both numbers are filed with the correct authorities,
and we highly encourage that you cross-register your numbers on both databases.

The Legal Language

The 1988 amendment of the SOLAS requires the outfitting of GMDSS equipment on ships subject to the convention.
These are usually the largest vessels, such as ships in excess of 300 gross tons.
Certain emergency services such as NAVTEX and the satellite EPIRB have been required on all SOLAS ships
since 1 August 1993, and all other GMDSS equipment was required as of 1 February 1999.

GMDSS outfitting is required for ships subject to Chapter IV of the amended SOLAS convention.
Covered are all ships engaged in international voyages except:

Cargo ships less than 300 gross tons
Ships of war and troopships
Ships not propelled by mechanical means
Wooden ships of primitive build
Pleasure yachts not engaged in trade
Fishing vessels
Ships being navigated within the Great Lakes of North America.

Under the U.S. Communications Act of 1934, ships subject to Title II Part II And Title II Part III of the Act
are required to outfit for GMDSS under FCC Regulation 47 CFR 80 Subpart W.
Subject vessels are all ships including fishing vessels, to be navigated in the open seas outside of a harbor or port, except:

Ships other than passenger vessels less than 300 gross tons
Passenger vessels having six passengers or less
U.S. government ships
Yachts of less than 600 gross tons
Vessels in tow
Ships navigating solely on any bays, sounds, rivers or protected waters within the U.S.
Ships being navigated within the Great Lakes of North America
Small passenger ships meeting the requirements of 47 CFR 80 Subpart S.

Also, on 20 November 1998, the FCC waived certain GMDSS rules applicable to fishing vessels and small passenger vessels.