It depends on how the slip ownership is structured. If the slip is just leased from the moorage owner, then you'd take an assignment of lease (which you'd want acknowledged by the moorage owner), which might need to be recorded as a real property assignment if the slip lease is a real property lease.
If the borrower owns the slip, it may be (or may not be) more complicated. Sometimes (at least in the Northwest) the state owns the riverbed under the slip, and issues a long term lease to the slip developer or condo association. The condo association sells individual slips to boat owners; the slips also require membership in the condo association. In that case, it may not be a real property transaction at all, but an assignment of the slip rights and condo association interest. It would need to comply with the condo association rules (probably requiring consent by the association) and you would want to do a UCC filing as well.
You might want to get counsel involved on this one . . .
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