As a seller, once you are paid and have a fully signed bill of sale you don't have much to lose from my perspective. It's the seller that may worry you might not pay the lien and/or you might be unable to deliver a clean title to the seller (Since you don't have it on hand).
You want to make sure the funds are good, vs. He pays the lien off with a hot check and takes the boat.
What I did as buyer in your situation, was I paid the lien off with a certified check, we mailed the payment together, signed a bill of sale, and I took the boat, all at that same time. The seller called the bank and confirmed the cashier's check was good before giving me the boat, and I confirmed he mailed it there, so he would release the lien.
The other thing we agreed on is that I held back $1,000 that were due to the buyer only once the title was free and clear and available to me, to ensure the seller had an incentive to pursue the clear title as the lien was cleared.
It happened, as in most situations, that both seller and buyer were decent reasonable people, (or at least he was and I pretended to be, and behaved like it) so in the end it all worked out totally fine.
EDIT: For your protection be sure to specify what warranty you are offering in your bill of sale. I'm not a lawyer but "No warranty from Seller, expressed or implied of any kind is provided. Boat sold as-is." goes a long way if for whatever reason something unhappy happens later.