Often the reason for requiring the
boat be documented is because that process creates a Title for the
boat. Ultimately, the lender simply wants to be listed as a lienholder on the title ensuring they collect the balance due on the loan at the time of
sale.
In the US,
registration is done by the state and not all states title vessels. Enquire further and find out if ultimately the lenders requirement can be satisfied if the vessel is registered in a state that issues a title, with the lender as the lienholder on the title.