1. You had a contract with the marina.

2. The fact you told them of a new owner doesn't change #1. Unless they agreed to that in writing.

3. They have a claim against you. You may have a claim against your buyer based upon the actual terms of your agreement with him (and not necessarily what you "understood").

4. If you call (what you see as) their bluff, you will also get to pay for their attorneys to sue you, as well as the court costs.

My suggestion is that you contact the buyer, but no matter what he says, you'd best pay them, get the carcass out of there and keep all of your receipts. That will provide the foundation of any claim you may have against the buyer.

Good luck.