First, let's clarify the difference between Documented and Registered.
When you buy a boat, naturally you will want some proof that you own it. That is either in the form of a State issued Title or a Federally issued USCG Vessel Documentation (a documented boat). You never get both. The USCG has minimum requirements to document with them. See here: USCG National Vessel Documentation Center, FAQ Page
Registration is strictly a State requirement to identify vessels that remain in their waters for a period longer than they identify. This varies highly, but 30 to 90 days is common. They do this so that they can effect regulation, track usage and collect taxes.
You will always have a form of title, and almost always have a registration as well, unless you are eternally transient or overseas.
Our boat is Documented with the USCG and registered in RI. However, I am not required to display any RI registration numbers, because I am Federally documented. The Federal Documentation number must be permanently inscribed aboard, but it not required to be visible. Your documented boat name and hailport are required to be visible and there are specific requirements for them (size and location).
Our dinghy does not meet the minimum size requirements to be USCG Documented and, therefore, is both State Titled and Registered. Therefore, registrations numbers are required to be displayed on it.
Some say that putting the symbol T/T (which denotes Tender To) and then your Documented boat's name will somehow attach it to the boat and avoid the need for titling or registering it. I've never found where that is actually permitted in RI, but may be in other state. My best guess is this is rumor that confuses the difference and the T/T only tags along with the boats title (documentation), not its registration.