The seller needs to apply for a replacement/duplicate title. Part of that application in Minnesota includes an affidavit that can only be signed by the person who is on the original title.
The Minnesota DNR web site has the applications and fees outlined.
Might be a "smokin' deal" on a nice boat, but the lack of title prior to the transaction increases the buyer's risk. There's a lot that could be going on there, including improper prior transactions and no title actually following that boat around, or maybe a lien on the boat that was never released. Might be all on the "up and up", but then again, maybe not. The buyer has no way of knowing for sure.
If it was me, I wouldn't buy it without seller having a title in hand.