I had the same problem except I have never owned a boat before. I ended up registering it at my dads house, which will also be the address on my drivers licence. My buddy and I plan on doing a lot of travelling so my insurance is also listed with that address even though it is a few thousand miles from where the boat is currently, or will ever be. I don''t get any mail (other than junk mail) so even having a P.O.Box is a waste of time and money for me.
It seems that no-one could give me a straight answer about this and none of them wanted a P.O. Box as an address. They sure make it hard to cut ties with land. As far as tax is concerned I have heard that Tennessee is the cheapest for registration and Kentucky has no tax on boats but I do not know for a fact.