When I bought a boat in the USA I used a UK DOT Form for the process..
As you are in the States I suggest you visit this site for your forms.. https://www.legalcontracts.com/contracts/bill-of-sale/
After printing off a couple of copies all you need to do then is go to either yours or the sellers bank and get the documents witnessed and then signed and stamped by the Bank Manager..
No need for lawyers etc;.. and acceptable to US Customs.
Assuming it's a documented vessel I would have them sign over the documentation, CG bill of sale and also a standard sales agreement that states sales price. This is for possible sales tax you may owe on the boat. As have been said, save the money and do your own documentation transfer.