Re: Document or register my vessel?
In Florida, at least, it is not a choice between documenting or registering. It is a choice between documenting with the USCG or getting a title issued by the state. You have to have one or the other. Regardless, you have to register the boat (though if it is documented you do not have to display state registration numbers).
Again, this is for Florida. Not sure about the rules in California. My point is that you have three different issues to deal with here--documentation, titling, and registration. How they work together, and what your choices are, varies from state to state.