Unlike motor vehicles, boats don't come with registration documents.
Some may come with proof of ownership in the form of a 'bill of sale' but many don't, so you need to obtain proof of ownership. Do as much homework as you can. Before making your decision, bear in mind the likely re-sale value of the boat you are considering. The more unusual it is, the more difficult it may be to resell later on.
Some second hand boats may be former hire boats. Be aware that former hire boats may or may not have been well maintained. They may well need a lot of money spent to get them properly equipped for your use, and they may well have had a very hard life.
See more about finding your dream boat.