By all means get the abstract of title. In addition to boatyard and
mechanic liens (filed or unfiled), there is also the risk that a tax
jurisdiction may have a claim on the boat that has not been filed with
the USCG. It is difficult if not impossible to discover all of the
possibilities, but at the very least you should have words in the
purchase agreement to the effect that "the seller represents that the
boat is free of any prior claims, and promises to defend/pay any prior
claims that arise after the sale." This is standard verbiage on many
broker agreements.

If you are "really" concerned, rename the vessel and document it with
an offshore corporate entity. That will make it very difficult to
trace but will complicate the documentation process, incur costs and
raise other issues.