There are a few things you can do
1. Is the boat USCG documented? if it is the coast guard can tell you if it has any leins on it.
2. Depending on the state in which the boat is currently there should be a title to the boat similar to a car (at least most of the states I have lived in) and again the title will state if there is a lein on the boat.
3. I realize some states do not title boats so that becomes a problem I for one would think any bank in such a state would require documentation to produce a loan for the purchase of the boat otherwise how can they prove they have a lein? I know of several friends who had to document the boats they purchased because the bank said so. If the boat is currently documented it can be looked up by boat name at
http://www.st.nmfs.noaa.gov/st1/CoastGuard/VesselByName.html
I realize this isn't much but I did recently purchase a boat out of my home state and dealt with a bank on the purchase of said boat. The bank lending me the money required a clear title from the other state to transfer the money to the broker. I was not required to docement the boat because the other state had titles like we do here in Ohio. I might offer one piece of advice and that is to document the boat upon purchasing then you have a trail saying you or whomever owns the boat with no leins or with a lein from xxx bank.
Remeber this advice is worth what you paid for it I am not a lawyer etc...
Also from the usgc site
You may obtain an Abstract of Title which will show all bills of sale, mortgages, and notices of claim of lien filed and recorded by the Coast Guard. You may request the Abstract by fax if you pay by credit card or may mail your request with the appropriate fee to the NVDC. To get accurate information you must supply the name and official number of the vessel.
the fee for that is $25.00 but is only works if the vessel is documented.
http://www.uscg.mil/hq/cg5/nvdc/default.asp