If the debtors are being cooperative, the DMV will assist you in putting your name on the boat title as lienholder.
If the debtors are not being cooperative, you have to either give up and lose the money or sue them. If the amount is less than $10,000 you probably want to file in small claims. Otherwise, you will have to file a lawsuit in district court. The litigation process is not impossible to figure out, but can become rather complicated depending on your situation. Once you have a judgment, you can do what is called a writ of execution against the boat. If they do not answer the lawsuit, the quickest this could occur is two weeks after the judgment enters (in small claims that would be about ten weeks from filing and in district court about six weeks from filing).
Best of luck. I would suggest enlisting the assistance of an attorney either way, but if they are cooperative, you might handle securing the debt with the boat on your own with the assistance of the DMV. Otherwise, I strongly recommend you obtain an attorney to assist you in the process of filing and pursuing a lawsuit. Their expertise will save you hours of time and help you avoid the many potential procedural pitfalls.
I could discuss this matter with you briefly with no fee if you wish. My number is 801-200-4937.
Providing this answer does not form an attorney-client relationship. Most legal questions are exceedingly fact-sensitive and therefore this answer is a best-guess based on the information you provide. You should consult an attorney licensed in your state to further discuss your matter.