I'd probably approach this in maybe 2 different ways. Neither are good, I'm just thinking out loud. 1st, I would consider offering to purchase the boat back from the guy less the slip rental fees in the contract. Id make sure I'd get the title back and make sure the title is in the same condition as it was given to him. You could include the cost of the additional slip she had to rent, because he didn't move his boat, but that might be a deal breaker to him, but then again it allows him an out either way. Then resell it with a more binding contract to get the boat removed by the future owner. Doing this would end the nightmare to a point, but she now has 2 boats and 2 slips. 2nd would be to take him to small claims court. Typically there are no lawyers involved. Sue for past due slip rental and the cost of the 2nd slip, and any other money you think you can get away with, and ask that judge to require the new owner to remove the boat from the slip. Essentially, a court order. In the OP's 1st thread they say she has a contract for the slip rental with penalties and a signed bill of sale from the new owner, showing she is no longer the owner of the boat, This can go along way towards the shift of responsibility & removal of the of the boat. I don't know about Missouri, but most states require the current title holder to sign the title allowing the boat to be transferred to the new owner. IF you have a judgement in your favor and doesn't pay, then have his wages garnished. A lot of employers don't like it when their employees have their wages garnished. You might be able to get a lien on the boat or on his home also. Usually, If he doesn't show up to court the judge will find for the plaintiff.
Legally, I don't think he can sell the boat when its not in his name. I know its been done b4, but finding that person to buy the boat that way will be tuff. He'd have a lot of xplain'n to do.