I can understand why the new "owner" would want to conceal the property, he brought it he thought "fair and square" and when he was contacted by the marina saying the bank was coming to take it, he saw his $115K or how ever much he brought it for "floating" away, its a natural human instinct, doesn't make it legal though.
I was in a similar situation with a third party take over, it wasn't a straw purchase because at one time the car i owned was for me, but then the third party disappeared with the car, and left me holding the balance w/ no car to sell to recoup, I couldn't pursue it through the police because it was a business contract because the car wasn't stolen, I put it in the program. But I was able to sue them for the damages to my credit report and for fraud, problem is I could never find them to serve them and I still ended up footing the bill.
In your case the boat was stolen by your friend and illegally sold to a third-party, you can't collect damages from the third-party because they transacted in good faith, but you can sue and possibly press criminal charges against your friend. As far as the third-party is concerned they can't withhold a boat that they have no legal rights to, its sucks for them.
What I would do if I was you is contact the third-party, either personally or through a lawyer, both in writing to support your communication and over the phone, and explain to them the situation. Have them return the boat to you/the bank, and then begin researching how you both can sue your friend. Problem is its not the bank's fault that you did a straw purchase and you are the ultimate one responsible for the balance and they will win any lawsuit they file against you, you could file a cross complaint (third-party complaint) but there was no contract between your friend, so the Court will not entertain that your friend owes the bank and not you. This means its a separate T/O so you will more than likely have to foot the bill, pay the bank, and hope to find your friend and try to get your money back.
Since it is more advantageous for the third-party to sue your friend, I would try and add myself to a lawsuit they would try and file against your friend, and split court costs and legal fees in a fair proportion to the damages sought.