Classic scam. This script has been around for as long as Craigslist. The fake “buyer” is always either “in the military” or an “engineer/oil rig worker/project manager” working overseas so you never meet or talk to them in person or on the phone. Sometimes they will send you a Faebook name/account but those are always faked too (note the lack of “friends” and posts on them.)
The “cashier’s check” that the “buyer” will send or the “shipper” will bring you will be accurate looking but totally counterfeit AND it will be for a higher amount than you agreed to sell the boat (or car or bike or diamond ring or whatever) for – the “shipper rep” will claim that was a mistake that they have no control over and they will ask you to cash it at your bank and refund them the difference, usually $1000 or more. If they get the cash from you (if your bank falls for the faked check and sometimes they do until it comes back as fake and they debit your account) chances are the fake “shipper rep” won’t even come back for the boat though they will claim they will do so later. If they do take it you will be out both the boat and a bunch of money.
These scams mostly originate in Africa or Eastern Europe. They are surprisingly successful, people are gullible.
If you want to yank their chain, give them a fake name and address and tell them to send the check there. At least they will waste time and money for a little while in which they won’t be seeking dupes that fall for their scam. One of my friends did that a few years ago when the same scam was sent to him regarding a motorcycle he had for sale. He told them to make the check out to “Ima Phroddster” with the address of the regional FBI field office (here is a link to their locations if you want to use one.)