Most people feel that if you are headed offshore, it's a nice idea to have some document that shows your boat is registered with the US Government, and not just your local state. To do that, you can register your boat with the US Coast Guard, after which you receive a fairly impressive looking document. The idea being that a local offical in Belize will be more impressed with a US document than he will be with a Michigan state registration certificate. The downside is that the registration process involves some forms that are quite difficult to fill out correctly - so much so that many people (like me) end up paying retired coasties to document their boats for them.
In my case I documented my boat because the bank required it as a condition of the loan, since having the boat documented allows the Coast Guard to trace a lein on the boat. The only other benefit is that, in most states, if your boat is documented you do not have to place those ugly state registration numbers on the bow.
There is a persistant rumor, especially here in Michigan, that if you document your boat you do NOT have to register it with the state. If you buy your boat out of state, and haven't paid sales tax on it, not having to register it with the state can save you 6% of the value. But this is NOT true, and though people still do this, the practice is not legal.
Is it worth it? I sure wouldn't have bothered if the bank hadn't made me, since I haven't gone offshore with this boat. But if I were going offshore I definitely would.