Maryland, like many states, requires that you register your boat within 30 days of purchase. Upon registration, you are required to come up with a big chunk of money based upon the purchase price, or the blue-book value, whichever is highest.
A friend recently purchased a Documented boat that had an outstanding lien. Because the lien had not been satisfied prior to the purchase, the bank claimed they could not release the title until all the paperwork had cleared. After weeks of calling the bank and asking for the title, the bank said "We don't have a title for the boat--it's documented and you will have to obtain documentation from the Coast Guard, a process which took months.
Because of the delays, when the owner(s) went to Maryland's DNR office to register the boat, they not only had to pay the 5-percent tax, but additionally, $219 in penalties and interest. DNR said we don't care about the delays in documentation, the only thing that counts in this matter is the bill of sale, which was dated 5 months earlier.
From my perspective, I don't believe they legally owned the boat until either the title, or U.S. Coast Guard documentation papers, were received. They've sent a letter appealing the fines, but in Maryland, the process of getting through the bureaucracy will probably take the rest of their lives before it's resolved.