This situation happens all the time. Most boat that are sold, have liens on them. Not sure how RI works, but in most states, as long as the new owner has a Bill Of Sale, or contract showing that they just purchased the boat, they should be fine. They may be able to get a Temp from the DMV. If the purchase was doen through a dealer, the dealer would provide a Temp Registration so the new onwers could use the boat asap. Check with your local DMV. Assuming the new owner is going to register and title in RI again, this link may be of help..
https://www.ri.gov/dem/boatrenewal/
His isurance company shouldn't need much besides vessel info, and maybe a copy of the bill of sale from you to him. Do yourself a favor and DO NOT cancel your insurance until you recieve confirmation that the payoff has been made. The boat is still yours until your loan is paid in full, and you can confirm that with the bank.
not an expert on how RI works, but this info should help you