Cashier's checks can and are forged very easily. However a call to the bank to verify the check is legitimate is also very easy. So with a couple of precautions checks can be taken without much issue.

That being said in this day and age of electronics why not just wire transfer it? I don't want cash, its to hard to manage and easy to lose. I certainly don't want to be at the interstate rest stop counting out 45k in 20's while a) the highway patrol is watching wondering what the hell is going on or b) crack head in the car next to us is wondering how far we can go before we will need to stop for gas so he can whack me in the head.

Another option is if they are financing the boat (most likely) then their loan officer will gladly send you the check directly from the loan proceeds in exchange for you sending the paperwork he needs for the transaction. ( I do this everyday, especially with boats because of the lack of titles) the seller usually has to wait an extra day or two but the guaranteed funds is usually worth it.

Just some ideas.