A Hull Identification Number (HIN) is an identifier meant to uniquely identify
  a particular vessel. They are the equivalent of a Vehicle Identification
  Number (VIN) on a car. No two vessels should have the same HIN. The HIN is
  filed with many different documenting agencies, including states, the U.S.
  Coast Guard (USCG) (or other countries' coast guard), insurance companies,
  banks, marinas, etc. So just as listing a wrong Social Security Number on a
  tax return is likely to cause problems, listing a wrong HIN on a form is
  likely to cause problems too.
  We have owned two boats. When we acquired them, the paperwork on both of them
  had a HIN listed that was different than was etched on the boat. Based on a
  sample size of two, this is a
  very common thing. If it is messed up, it is worth the trouble to get
  it fixed. (One had a misreading of the badly etched HIN, the other had the
  wrong build month recorded.)
  A couple of caveats in the discussion below...This discussion is about
  recreational boats built since November 1, 1972 sold in the U.S. I'm not going
  to talk about boats built before 1972, homemade boats, boats imported from
  foreign countries that don't follow United States rules, or commercial boats.
The Manufacturer's Identification Code (MIC)
  Each manufacturer of marine equipment, including boats, is assigned a three
  letter code that uniquely identifies them called the Manufacturer's
  Identification Code (MIC). For example, Catalina Yachts is assigned CTY. You
  can find the definitive list at
  USCGBoating.org. These codes are used throughout the boating industry, so if, for example,
  you visit the
  Fisheries Supply web site,
  their product descriptions includes these MIC identifiers.
Finding the HIN for Your Boat
  First, let's find out what the HIN is that is assigned to your boat. The HIN
  should be recorded in two places on the boat (however, the rules have changed
  over the years, so boats built before August 1, 1984 may not follow all of the
  rules). Here are the exact rules for where a HIN should be located from
  33 CFR §181.29:
  § 181.29 Hull identification number display.
Two identical hull
    identification numbers are required to be displayed on each boat hull.
(a)
    The primary hull identification number must be affixed—
(1) On
    boats with transoms, to the starboard outboard side of the transom within
    two inches of the top of the transom, gunwale, or hull/deck joint, whichever
    is lowest.
(2) On boats without transoms or on boats on which it
    would be impractical to use the transom, to the starboard outboard side of
    the hull, aft, within one foot of the stern and within two inches of the top
    of the hull side, gunwale or hull/deck joint, whichever is lowest.
(3)
    On catamarans and pontoon boats which have readily replaceable hulls, to the
    aft crossbeam within one foot of the starboard hull attachment.
(4)
    If the hull identification number would not be visible, because of rails,
    fittings, or other accessories, the number must be affixed as near as
    possible to the location specified in paragraph (a) of this section.
(b)
    The duplicate hull identification number must be affixed in an unexposed
    location on the interior of the boat or beneath a fitting or item of
    hardware.
(c) Each hull identification number must be carved,
    burned, stamped, embossed, molded, bonded, or otherwise permanently affixed
    to the boat so that alteration, removal, or replacement would be obvious. If
    the number is on a separate plate, the plate must be fastened in such a
    manner that its removal would normally cause some scarring of or damage to
    the surrounding hull area. A hull identification number must not be attached
    to parts of the boat that are removable.
(d) The characters of
    each hull identification number must be no less than one-fourth of an inch
    high.
  On a Catalina 320, the primary HIN is affixed on the starboard side aft, just
  below the rubrail. On the Catalina 22, it is on the transom just below the
  rubrail. In both cases, it was scratched into the fiberglass with an etching
  tool. On the 22, it was difficult to read partly because the handwriting
  sucked. The second HIN on the 320 is on the starboard side under the V-berth
  on the bulkhead, but on other boat models will be in other places.
Furthermore, § 181.27 says that:
  With the exception of the characters “US-”, which constitute the country of
    origin code for the United States, if information is displayed on the boat
    within 2 inches of the 12-character hull identification number (HIN), that
    information must be separated from the HIN by means of borders or must be on
    a separate label, so that it will not be interpreted as part of the hull
    identification number.
HIN Formats
  Since November 1, 1972 there have been three HIN formats: The Current Format,
  which has been used since August 1, 1984 to the present. And two older formats
  used between November 1, 1972 and July 31, 1984: The Straight Year Format and
  the Model Year Format.
The Current Format is described in § 181.25:
  § 181.25 Hull identification number format.
Each of the hull
    identification numbers required by § 181.23 must consist of twelve
    characters, uninterrupted by slashes, hyphens, or spaces, as follows:
    
(a) The first three characters must be a manufacturer
    identification code assigned under § 181.31(a) or the importer designation
    assigned under § 181.31(b). 
(b) Characters four through eight
    must be a serial number assigned by the manufacturer in letters of the
    English alphabet, or Arabic numerals, or both, except the letters I, O, and
    Q. 
(c) Characters nine and ten must indicate the month and year
    of certification when a date of certification is required. In all other
    cases characters nine and ten must indicate the date of manufacture. The
    date indicated can be no earlier than the date construction or assembly
    began and no later than the date the boat leaves the place of manufacture or
    assembly or is imported into the United States for the purposes of sale.
    Character nine must be indicated using letters of the English alphabet. The
    first month of the year, January, must be designated by the letter “A”, the
    second month, February, by the letter “B”, and so on until the last month of
    the year, December. Character ten must be the last digit of the year of
    manufacture or certification and must be an Arabic numeral. 
(d)
    Characters eleven and twelve must indicate the model year using Arabic
    numerals for the last two numbers of the model year such as “82” for 1982
    and “83” for 1983.
  
  So, for example, a Catalina 320 HIN begins with CTY. This is followed by a
  serial number begins with the letter "C", then followed by a four digit
  sequential hull number (very early 320s used Q instead of C, but Q is not
  allowed in the serial number as described in the rules above because it might
  be confused with 0). So hull #87, would be C0087. For the Catalina 22, the HIN
  serial number beings with the letter "H" followed by a four digit sequential
  hull number. However, Catalina has made more than 20000 Catalina 22s, so this
  has rolled over twice. If you see H2284 as the serial number, you will need to
  look to the last 4 characters to determine whether the hull number is 2284,
  12284, or 22284 from the HIN and doing a little research on when these hull
  numbers rolled over. Only a few boat models have had more than 10000 hulls
  built and have this issue.
  The month of manufacture is then encoded with a letter A-L. This followed by
  the last digit of the year of manufacture. So, October of 1993 would be
  encoded J3 (but so would October of 2003). The last two digits are the model
  year of the boat. Model years typically start at some point in the previous
  year. For the Catalina 320, the new model year starts in June of the previous
  year, but many manufacturers start their model year in August. Model year 1994
  would be encoded as 94, whereas model year 2004 would be encoded 04.
  So putting it all together, Catalina 320 hull #87 build in October of 1993 as
  a 1994 model year would have a HIN CTYC0087J394.
  Note that HINs can contain the letter I in the MIC and in the month of
  manufacture, and that is a different character than the number 1. Make sure
  that I, 1, L, O, Q, and 0 are recorded correctly when submitting any
  paperwork.
The older formats are encoded like this:
  Straight Year: 3 digit MIC, 5 digit serial #, 2 digit month of production, 2
  digit year. Example: CYT6A130579 for Cobalt Yachts with serial number 6A130,
  built in May of 1979.
  Model Year: 3 digit MIC, 5 digit serial #, the letter M, 2 digit model year, 1
  letter month of production. The key for the month in Model Year format is
  A=August, B=September, ... L=July. Example: ZVMD1769M80L for Vandestadt and
  McGruer with serial number D1769, built in July of 1980. Boats built a month
  later would have the last four characters being M81A.
What To Do When the HIN is Wrong
  When both of our boats had an improper HIN on some of the paperwork, we needed
  to get it fixed with the Coast Guard and the state. We first needed
  photographs and a rubbing of the HIN on the hull. Then we needed to present
  that info to the state department of licensing. For the Catalina 22, after
  paying appropriate fees, a new registration and title was issued. For the 320,
  the title company got the HIN straightened out with the Coast Guard
  documentation, and after presenting the Coast Guard documentation to the
  state, the state issued a new registration with the fixed HIN. The old HIN is
  recorded in a comment on the registration.
  The Coast Guard rules for a malformed HIN are described in
  this document
  (along with much other materials about HINs). It shows this flow chart for how
  to fix a HIN: