Vessels work just like cars in Florida. Titles need to transfer from one owner to the next. Joe can only sell you a boat that Joe legally owns (ie is titled in Joe's name). John, even if in possession of the boat titled in Joe's name, cannot legally transfer ownership - since he doesn't own it. If several owners passed the boat along without following these procedures, then the vessel is still legally owned by the last person to have the registration in their name. If you can't locate the previous legal owner, then as stated in DMV procedure TL-07, you will need a court order before a certificate of title will be issued to you. The court will want to see proof of purchase, and your documentation of what steps you took to locate the registered owner. Contact your local Clerk of Court for their procedures to bring your motion before the court.
THEN, be prepared for the court to ask if you've had FWC issue you a Certificate of Compliance (cost: $300) which will show the court that (a) law enforcement certifies that the vessel hasn't been reported stolen, (b)that FWC has made a good faith effort to locate the previous owner, and (c) that a vessel inspection was done and the vessel was deemed seaworthy.
If you skip those steps, the odds of the court ruling favorably on your motion start to decline rapidly - in which case you're not only out the money, but in possession of a vessel that you can't get registered; the remedy being to sue the person who sold you the property improperly in the first place.

