I was forced to sign a paper for payment of $1800 to a boat yard. Can I get out of that payment?

There is a legal doctrine known as economic duress which provides that contracts entered into under economic duress are void.  However, it is very difficult to prove and usually does not succeed as a defense.  The party claiming duress has to show that the other party was threatening to do something that he/she had no legal right to do, which would have caused imminent harm to the party claiming duress, and that the party claiming duress had no alternative but to sign the contract.  The leading case involved a defense contractor with a government deadline by which he had to complete a project or face substantial and ongoing penalties.  One of his suppliers raised the price of supplies, despite a contract fixing the price, and no other suppliers were available.  By the time the contractor could have won a suit, the deadline would have long passed.  The Court held that the contract for the new price was procured by economic duress and void.  You might be able to get out of the contract you signed the same way, but, again, Courts are generally not sympathetic to claims of economic duress and you might not win.