It's your boat, and you have the documentation to prove it, so legally it is yours.

Your "friend" did not have title to the boat and the (innocent, but foolish) buyer should have checked this before parting with the money - Caveat emptor and all that.

Having said that, don't just go and take it, involve the police and get them to go with you to avoid any unpleasantness with the buyer. He will be rightly hacked off and will be out of pocket. Whether he can reclaim the sale price back from your "friend" is down to him and may not be easy to do.