That depends on the state. Some states still require that you pay the sales tax, as well, in spite of the boat being documented. Check with your local Department of Revenue, Department of Motor Vehicles, or whoever handles boat registration. In our state, registration and sales tax is still due on a documented vessel kept in the state.
For financing purposes, some lending institutions require documentation, because there is a very clear paper trail of ownership. For the Bahamas, Mexico, and Canada, those countries see plenty of US vessels that are not documented and know what to do with state registrations.
I've heard of some people who try to avoid paying sales tax and/or state registration by documenting. This will vary by state; some states require that you pay registration even though your boat is registered in another state, IF your boat is in their state waters for a specified time (i.e. 30 days, 90 days). We had a sailboat that was documented, and when brought into our state, the local Game, Fish, & Park folks would constantly stop us because of having no registration numbers on the boat... explaining documention got blank stares from them and more hassle... easier to put state reg #s on and not get stopped every time we went out.
Re: the documentation numbers - they need to be permanently affixed to the boat in such an manner that removal would deface the boat. I put vinyl numbers on a storage compartment wall and then went over them with epoxy.
Best wishes,
Jim B.