Ron n Kelly,
A good skip tracer may be of assistance. An automoble one will work as well or better than any PI. Call your bank, or a big bank (like B of A) and ask for references. they use a lot of repossesors and even tho the banks are a bit dense in a corporate sense, they can be of assistance.
Keep your eye's and ears open when dealing with repossesors, they are used to dealing with some slippery characters and often have adopted a similar attitude. It would probably not be nessesary (or even advisable) to hire them to reposses the vessel for you. Just get them to skip trace for you.
Keep in mind that runners will often leave a trail of bad debts at Marinas, fuel docks, boatyards, resturants. Any bank #'s, credit card #'s references, former addresses etc. can be of help to a good skip tracer.