Thank you for your question. I look forward to working with you to provide you the information you are seeking for educational purposes only.
I am sorry to tell you, but you bear the burden of proving ownership and if you have none of the documentation proving ownership, you have the responsibility for obtaining it. If you never registered them initially or transferred title to your name when you obtained it from the finance company and the seller has no records (you need to speak to a supervisor), the sole remaining option I am afraid requires you hiring a local attorney and filing a "complaint to quiet title" in the court and serving the last registered owner and finance company a copy of the suit. Once you do so, you have to then bring in any bank or payment records to court to show you paid for the boat and the court has to issue an order to issue title in your name.