I've bought 4 boats over the years never worried about deregistering them. Never been a problem. 2 of them I have taken to Oz. Customs never asked for a deregistration paper. 2 of these boats we sailed back to the US at times.
Now, having said that the most recent boat I left from California and had a problem with the self steering gear so we made a stop in Honolulu. US boats and citizens do not need to clear in or out but we had to not being that!!! I walked to customs and the lady there asked for the deregistration papers. I told her that we've never bothered before. She was friendly but took the boats papers and would not return till we had the correct paperwork. Had to walk up the street and pay an agent $120 and he did it very quickly for us.
When you buy a boat in the US you'll get a bill of sale with a sentence saying that you paid $1 and other valueable considerations. This is to avoid paying state tax depending on the state..... they are all different. John