There's a lot of confusion over whether - and when - a Coast Guard Documented vessel needs to be registered with the state. The short answer is that UCSG Documented boats never need to be titled with the state, but may need to be registered with the state depending on how long the boat is on the state's waters.
First, as to the titling of the vessel, a vessel may either be state titled or federally documented, but not both. Think of USCG documentation like your title - that's really what it is, even though it has another name. In fact, federal law under 46 U.S.C. § 12106 actually prohibits a USCG Documented vessel from being state titled, and a state cannot require a USCG Documented vessel to display numbers.
However, states may still require registration of USCG Documented vessels for tax and other purposes. What's the difference between the title and the registration? The title is a formal, colored document used to prove and transfer ownership. The registration typically consists of a registration card and an annual state decal which must affixed to the boat. So while the Coast Guard Documentation is your boat's title, you may have to also register with a state.
Registering with a state is normally required if you keep your vessel in a state for a certain amount of time. For example, registration is generally required in Florida if your boat spends more than 89 consecutive days in the state. The Sarasota Tax Collector has an excellent guide for how to register a USCG Documented vessel in Florida, which states that to get your registration and Florida decals you must bring the following:
Application to Register Non-Titled Vessel
Copy of documentation papers
Copy of executed bill of sale to document amount of sales tax due
Proof of payment of sales tax or proof of tax exemption
$50 non-resident commercial vessel fee, if applicable
So while USCG Documentation exempts your vessel from having to be state titled or display state numbers, if you keep your vessel in a state for a long period of time you may have to register and/or pay tax. If you need state-specific advise on tax and registration, contact your local state titling agency or a maritime lawyer.