Presuming you have access, and can remove the nut, make op a grommet of caulking cotton and put it under the washer. Add a little Butyl rubber sealant (not sika or silicon) and replace the nut. Wind it up as tight as you can.
To avoid disaster (a spinning bolt), cut any excess thread off BEFORE you remove the nut to save it binding on the rust.

A larger plate washer may help.

A masic injection via grease gun and nipple into the bolt void would cure it, and can be done between tides. However you do need the gear for this.

Did you not check all c/line fastenings during your restoration??