With houses and boats (and probably lots of other things) you can almost never be absolutely certain that the title is clear. That's why you get title insurance. It doesn't guarantee that the title is clear. What it does is guarantee that you will be compensated (get your money
back) if later on it turns out that the title is NOT clear. Of course, before offering this insurance a reputable title company will do all it can to search the title and be sure that it really is clear--they don't want to have to pay out on the insurance if they don't have to.
Anyway, back to the original question, how could this happen? Pretty easily, really. It could be that there has been a lien on the boat for years. Or it could be that the boat was previously stolen, sold, resold, and only now have the rightful owners found it.
Think about it. If you had a boat stolen from you 20 years ago, and just yesterday--through some miraculous coincidence--you saw it in your local marina, what would you do? You'd go get the cops and you would repossess the boat. The fact that the current
owner THOUGHT that he bought it perfectly legally 15 years ago wouldn't really matter, would it? It wouldn't make you stop and say, "Well, after all these years, I guess I should just let that guy keep the boat."
Whether it is a good idea to get title insurance or not is up to you. It is like any insurance: you get it to cover a loss that you would not want to cover yourself, but you always hope that you never actually need it. So, how sure are you that the title is clear? Could you afford the loss of the boat if it turns out that the title is NOT clear? If the answer to the second question is "absolutely not" then (unless the answer to the first question is "100% certain") you had probably better get the insurance.
In any case, while a boat being repossessed 10+ years later is not a common thing, I'm sure this is not the first time it has happened.