Bill of sale??
Depending on your state law, a bill of sale will yeild a title in many states for the boat, if the serial number was recorded on the bill of sale. Some states will require that there be a notarized statement from the previous owner that the boat has not been used for "X" number of years if it has not been registered in a while. I agree with "trailer" response above in most cases. Some states require that you supply invoices for the materials that you built the trailer with before issuing a title and tags. Instead of guessing, why don't you call the licensing agency and explain what your circumstances are and usually they are more than willing to tell you what steps are required to get the titles for your boat and trailer. Since each state has different regulations regarding the securement of titles, it is just a guess unless someone from your state, that is familiar with the laws, responds to your question. Also, if you contact your state agency and they give you the requirements those will have to be followed or you will not be able to obtain what you are looking for.