OK, lots of misinformation in this thread.
A title is an ownership document issued by a state. You can physically "hold" the title in your hands, but that's inconsequential because you can easily obtain replacement copies. You can legally hold title - i.e., the boat is titled in your name, making you the owner of record - but that doesn't mean there aren't interests not reflected on the title document.
Many of the posters are rightfully concerned that you're getting scammed. You haven't explained how you documented the sale of the vessel. There are innumerable scams involving titles - at the end of the day it's a question of proof (when you sue the seller for your money, how you will prove that you actually paid the money).
There's also the issue of liens. Responsible lenders record liens on the title document (so they can ensure they get their money at any sale), but that doesn't mean there aren't unrecorded liens. Still, reviewing the title document during a purchase is a common sense step.
So, review your paperwork for comfort. Don't rely on conversations or other unwritten agreements. Verbal contracts are generally unenforceable for a reason - there's no way to reliably prove their existence or terms.
The problem here is the marina owner, who really needs to spend some money on legal counsel. Calling him an idiot is being charitable. There are processes for getting rid of abandoned vessels (often at a profit) and for recovering payments from deadbeats (with costs and interest). His conduct as described by you is just bullying, and doesn't help anyone.